NEW YORK, JULY 14, 1893.

THE WRENS OF TRAVIS COUNTY, TEXAS.

BY CHARLES D. OLDRIGHT, AUSTIN, TEXAS.

1. Catherpes Mexicanus conspersus, Cañon Wren. This bird is an "endemic" species, its occurrence in any district depending on the topographic features. The great rock walls of the Colorado River, and the numerous side cañons form an ideal dwelling-place for this little bird, and here it may be found at all seasons, and its loud, ringing song re-echoes from cliff to cliff in the dreary days of November as well as in April's sunshine. But it penetrates into the city, and every morning this year one of the first sounds that I have heard has been the matutinal song of a cañon wren whose nest was in a cranny of an unoccupied house standing next to mine.

The cañon wren (as active a busy-body as the rest of his tribe) seems to be never too tired to sing. Reclining on the soft grass at the margin of the rivulet you look up the great frowning cliff and see a tiny bird, now clinging to the perpendicular rock, now disappearing in some crevice of the cliff and then perching on a projecting fragment, he utters a succession of clear bell-like notes in a descending scale.

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As this wren usually nests in some crevice far up in the cañon wall its eggs are often safe from the hands of the oölogist. Many times have I gazed longingly at a few straws projecting from a hole, while the owner of the nest watched me complacently. In such cases 'tis distance lends enchantment to the view." However, I have had the pleasure of examining several nests containing eggs and young, and as I have never seen any detailed account of the nidefication of this species, I will give some particulars about them.

This bird begins building early in the season, a nest with hatching eggs in it having been taken on the 30th day of March. In 1890 fresh eggs were found April 3, 4 and 11.

The nest is placed in some cranny or hole of convenient size, always in the face of the cliff; other situations are on a rafter in a barn, under the cornice on a veranda and in the chimney of an uninhabited house.

The nest is composed of grass and weeds and lined warmly with hair, wool and cotton. The complement of eggs varies from three to five, four being perhaps more usual than either of the other numbers.

The eggs always have a clear white ground, while the markings vary from a very slight sprinkling of brown pin-points to numerous large blotches and spots of reddish-brown and lilac, forming a confluent ring encircling the crown; this is the most common pattern of coloration. Their shape is ovate or rounded ovate, but I have seen one pyriform egg in the nest with three other normally shaped eggs.

2. Phryathorus ludovicianus, Carolina Wren. An abundant bird in the bottom lands along brooks and in all heavily timbered country. The Carolina wren is another fine singer, but spends too much of its time in scolding owls, crows and men. But often, especially in the spring and at sunset on a summer's day, one of these birds will perch on the topmost twig of a tall shrub and will, with his tail drooping and head thrown back, call "sweet William " until the woods resound. By the way, "sweet William " does not express the exact sound of the bird's notes to me, but I am so hopeless of expressing birds' voices by English words that I will not attempt to amend it.

This bird cannot be called particular in its choice of a nestingplace, for their nests have been found in hollow logs, under the cornice of a house, in a tin can placed in a tree, in a hole in a rock wall and on the window-sill of a farmhouse. The hollow log is, I believe, the most usual situation. The nest is made to fit the cranny in which it is built and generally fills it. Twigs,

grass weeds, leaves, hair, cotton, wool, rags, paper and even other materials enter into its composition. In shape it is more or less rounded, with an entrance in the side. The eggs are four, five or six in number, five being most common.

There is not much variation in the eggs; the markings being in some heavier than in others. The ground color is white, spotted thickly and finely with specks of reddish-salmon color and lilac, generally forming a poorly defined ring around the crown. The ground color is usually well concealed.

Fresh eggs may be found from April 1 to May 15, the height of the breeding season being during the middle of April. 3. Thryothorus bewickii murinus, Baird's Wren. Probably our commonest wren, found in all kinds of country, bottoms or uplands, forest or prairies, mountains or plains. I believe, however, that Baird's wren prefers a broken country, little patches of prairie and mesquite groves alternating with the timber.

A number of these birds must spend their whole lives in the city of Austin, for in nearly every garden one may find a pair.

They are fussy little creatures hardly ever silent for a moment but keeping up a lively chatter or querulous “chee,【chee, chee.” But all through the spring, even as early as January, the males are great singers, and early on an April morning one cannot go far without hearing the sweet and cheerful song of one of these little birds. At such times one finds the bird perched in a tree-top, but on other occasions he will be hopping amongst the bushes or along a rail fence, flirting his long tail, uttering a continuous "chirp, chirp," and at each third "chirp" stopping a moment to pour forth his little ditty. This is kept up for hours at a time. In February the wrens become restless and may be seen promenading the back yards in pairs, peering into every hole and bird-box. They seem to be often undecided as to a nesting place, for I have known of a pair starting four different nests within a week, without any apparent cause for their fickleness. Any place seems good enough for this bird to start a nest-though as I have just stated they are more particular about its final location. Many people here have put small wooden boxes at their gates for the reception of mail matter, and I verily believe that each one is looked into once a year by a Baird's wren, with a view to building in it, and indeed many are chosen as nesting sites.

The nest is simply a mass of rubbish—but always warmly and softly lined with feathers or cotton. Six is a common complement of eggs, but as many as eight or as few as four may constitute a full set. The eggs are white, more or less speckled with brown of varying shades and lilac, sometimes the specks of reddish brown are thickly and uniformly distributed, again they are collected into a ring surrounding the crown or else rather larger specks of chocolate brown and lilac shell markings are more sparingly disposed.

Two "albino" eggs came under my notice last spring; one was immaculate white, the other had a very faint speckling on the crown; both these eggs were with other normally colored eggs.. I also found a peculiar "runt" egg of this species, it is of normal coloration but measures only .55 by 44, being thus the size of a humming bird's egg. I found it one day in a hole in a telephone pole, and left it thinking that more eggs might be laid, as I saw the birds at hand; but when, after the lapse of several days, none were deposited, I took it. Why the bird laid no more I do not know. Surprise at the first one may have had something to do with it.

4. Troglodytes aëdon aztecus, Western House Wren.

Of this member of the family I can say but little, for during his winter stay with us he is very silent and indeed shy.

I am aware that he, like his kinsmen, can scold with remarkable vehemence, for I have heard him. While he remains with us he is to be found in the creek bottoms wherever there are

thickets of brush-wood. He remains with us until late in the Spring, indeed the other wrens have young ones before he thinks of leaving for his northern summering place." Last year I saw some on the 22nd of April. I sent one of them to Washington where the "bird doctors" pronounced it "aztecus."

5. Salpinctes obsoletus, Rock Wren.

This bird hardly deserves a place to itself, being quite uncommon and differing little in appearance and mode of life from the Cañon wren, which seems to represent it with us. It is more common further west. Indeed, this is the most easterly record in Texas of its occurrence.

METALLIC CARBIDES.

BY F. P. Venable, chaple Hill, n. c.

Tuis name is given to compounds formed by the direct union of carbon with the metals. They are not numerous nor do they seem to be easy of formation and it is very difficult to prepare them in a pure and definite form. Consequently they have been but little studied so far. None of them are known to occur in minerals of terrestrial origin.

Interest in these bodies has been heightened of late by the discovery of new ones, and by the instructive decompositious of some of them.

First as to the general mode of formation. They are usually formed by the action of intense heat upon the metal in the presence of carbon. The form of this carbon is capable of being greatly varied. Graphite, amorphous carbon and many hydrocarbons can be used. The carbide is especially formed when the metal is being extracted from its compounds, that is, in the nascent state. Several metals thus unite with carbon in the process of manufacture, as zinc, copper and notably iron, and the presence of the carbides renders the metal hard and brittle. The purification and analysis of these bodies is not at all an easy problem, and hence little or nothing is known of their formulas or chemical constitution Five or more formulas have been assigned to iron carbide, and, of course, several may exist, still the correctness of any of these formulas is questionable.

The heat of the ordinary furnace is sufficient to form the carbides of the metals already mentioned. For others, more recently discovered, as the carbides of aluminium, of calcium, of barium, etc., the intense heat of the electric furnace is necessary. The first of these, aluminium carbide, is a most interesting body, of a light golden yellow color, it can be gotten from t'e electric furnace in a mass of corundum and metallic aluminium. It was described first by Sterry Hunt. Though it will stand

intense heat in the air without appreciable change, yet really it is undergoing change all the time as is proved by the oder of hydrocarbons coming from it and the fact that left to itself in air it crumbles in a few weeks into a mass of white alumina. A few shining golden scales of the pure substance can be separated, but so far no analysis has been given to the world.

All of these carbides, under certain conditions, give off their carbon in the form of hydrocarbons. The same smell can be detected in all during their decomposition. In some cases, as iron and zinc, the decomposition is caused by the action of an acid. The carbides of the earths decompose in moist air and more rapidly in water. Calcium carbide decomposes the most energetically of them all. The evolution of the hydrocarbons would be called violent. Of course, the hydrogen needed for the reaction comes from the decomposition of the water or from the hydrogen acid.

A

A most interesting fact recently published in the scientific journals, is that the calcium carbide on decomposition yields lime and pure acetylen gas. The acetylen seems very pure. thousand cu. cm. of the evolved gas was passed into an ammoniacal solution of copper chloride, and not a bubble went through. All was absorbed and precipitated. This is very important because the modes of preparing acetylen in common use are tedious or expensive, and hence this important hydrocarbon has not been as carefully studied as it otherwise might have been. The formation of hydrocarbons by the decomposition of iron carbide has furnished a basis for one of the theories as to the origin

of petroleum. If great quantities of iron carbide existed beneath the earth's surface and were subjected to decomposing influences, such oils and gases as are found in petroleum regions might very easily be formed.

So far there has been little utilization of these carbides commercially. One of the purer forms of iron carbide is used in a process for preparing metallic sodium, and the iron carbide in cast iron confers upon it many of its useful properties. If these bodies can be produced cheaply enough, however, there is strong probability that certain of them will prove very useful.

PHILOSOPHY IN THE COLLEGE CURRICULUM.

BY HOLMES DYSINGER, CARTHAGE COLLEGE, CARTHAGE, ILL. STUDIES under the name of philosophy are to be found in almost every college curriculum. Either because the subject is too vague cr abstruse for the comprehension of the average student, little more than elementary sychology, which is rightly regarded as a necessary part to the introduction to the subject proper, and a brief discussion of practical ethics are taught in most of the schools outside of the few real universities. While the number of subjects advocated for introduction into the college course is increasing constantly, one so fundamental as philosophy should not be neglected. Apart from its theoretical value, it has practical bearings upon the intellectual range of a mar, regardless of the system he adopts, that commend it to the thoughtful consideration of educators.

The subject-matter with which philosophy deals bears a peculiar relation to all other subjects in the course, in as much as its office is, partly at least, to systematize and explain all the principles of the particular sciences. This gives the unity so desirable in a course of study, and so essential to the thoroughly-trained mind. From this it serves the highest purpose in education and deserves a prominent place in every course of liberal culture.

The philosophical powers of man are last in order of development. The subject-matter makes it necessarily so. It is the most abstruse of all forms of knowledge. The mind in its unfo'ding passes up through perception and conception to the realm of widest generalizations and the discovery of the principles that are assumed in all our thinking. Philosophy deals with forms of knowledge that stand at the farthest remove from that furnished in so-called presentation the first development in the mind's unfolding.

When the mind reaches that stage of development in which it apprehends the principles fundamental to all knowledge, it turns in upon itself and critically examines its own processes and assumptions to determine the certainty of being and the validity of our knowledge. This is the highest stage in man's intellectual ascent. Here he stops. He has completed the circuit of the globe of knowledge. He started with the facts furnished in sense and consciousness, and ends in the principles that underlie and embrace all knowledge. These stand accredited in his own thinking. Beyond this the mind of man cannot penetrate.

That many students cannot attain this stage of knowledge is evident to all who have taught the upper classes in our colleges; that but few who attempt it get further than the outer court. is to be expected; but that all are greatly benefitted intellectually would not be denied by those who have looked into the merits of the case and examined the evidence with impartiality. A few additional facts will give our reasons for this conclusion.

Notwithstanding its abstruseness, as a discipline in thinking and in logical method, philosophy has no equal. Facts as furnished by the senses and distinguished from principles are not dealt with in philosophy, but the relation of facts to one another and to all things else. All these in a system of philosophy deserving of study or worth elaboration must be included in their relations of coördination and subordination. The unity of all bing is the ultimate problem of philosophy. A narrower range and lower ideal may satisfy science, but it cannot attain to that which comprehends all know ledge. Only the mind well disciplined in logical method can grasp the facts, but the one who attempts to do so will develop a power that is the possession of few and the desire of all.

This apprehension of facts as related is essential and necessarily precedent to the discovery of principles which govern these relations. In this respect practical fruit is to result from the study of philosophy. Not simply philosophers, but even the students of philosophy, must get a more comprehensive grasp of facts and principles, as each is assigned its place in the whole system of knowledge. Truth is apprehended in its harmonies and wholeness. It is seen in its proportions.

If more attention were given to a careful study of philosophy as a system, rather than in its history, much of the conceit of knowledge which is so prevalert to-day would be unheard of. The specialist would soon discover that he was occupying a very - small niche in the universe of knowledge; the broadest scholar that his horizon included but an infinitesimal portion of the sphere of truth.

THE Peak district of Derbyshire, so justly famed for its scenery, possesses also many attractions for the archæologist, among which are two stone circles.

The larger of these, called Arbor Lowe or Arbe Lowe, is about six miles from Bakewell, and consists of an oval ring, the diameters of which were about 126 and 115 feet, the precise lengths being difficult to ascertain in consequence of the stones, which doubtless originally stood upright, being now all flat, and having fallen, some outside, some inside, and some across their original positions, while others are broken into fragments or buried in the soil. There were perhaps about forty stones, of which nearly thirty remain entire or in fragments, the largest being about twelve feet long, six broad, and four thick. The longest diameter of the oval ran nearly northwest and southeast, and somewhat more to the west and east, two of the stones seem to have stood back outside the regular line of the oval. Within the oval, and on the line of the longest diameter, but not in the centre of it (the distances from the northwest and southeast ends being in about the proportion of three to two), are the re nains of some large stones - one fourteen feet long-which were apparently three in number, forming a "core," |_, like that in the centre of the northern circle at Abury, the central stone of which faced the rising sun on Midsummer Day. Like the circles at Abury, the stones at Arbelowe are surrounded by a ditch, which is about seven feet deep and fifteen wide at the bottom, outside of which is an embankment, formerly perhaps ten feet high and eight wide at the top; Sir G. Wilkinson says somewhat more, but it may be that he took the maximum and I took the minimum of the measure. This embankment is now very irregular, and in one place a tumulus has been formed from the materials composing it, in which were found two Celtic vases and a bronze pin. This tumulus could hardly have formed part of the original plan of the monument, and would therefore seem to have been made after the latter had fallen into disuse. The embankment, like that at Abury, is not a true circle, and there is much similarity in the irregularities of both, but that may be quite accidental. There are two entrances, one southeasterly, in the same direction as the Kennet entrance at Abury, and one to the northwest, but not quite opposite to the other; altogether Abury and Arbelowe, notwithstanding the great difference between them in size, have more points in common than any other circle has with either. Just outside the southeast entrance are two small stones, quite as likely to have been taken from the interior as to be in their original places. Nearly three hundred yards to the southwest is a tumulus, called Gib Hill, about twenty feet high and as wide at the top, in which a small cist was found, two feet under the surface, which contained a vase, two worked flints, and an iron fibula with places for stones - probably a secondary interment. A bank of earth of doubtful antiquity runs from the embankment for some distance in a direction south of Gib Hill. These various

1 No. 1, Abury, appeared in No. 529, March 24; No. 2, Stonehenge, appeared in No. 537, May 19. To those who may wish for more minute details of measurements than can be given in a shor: article, I would recommend "Stonehenge," by Professor Flinders Petrie, D.C L. (St nford, London).

earthworks have been supposed to give the form of a serpent to the monument, but Sir Gar lner Wilkinson's plan shows this idea to be quite incorrect; this is a point for the visitor to verify.

On the moors at the top of the hills above Eyam is a small circle of a different character from Arbelowe; it is called the "Wet Withins," and consists of a bank of earth, about six feet wide and two high, inside which, but close to the bank, was formerly a ring of small stones about two feet high and of proportionate size, of which ten remain, out of perhaps twenty or more. The diameter of this circle is about one hundred feet, and some sixty feet to the north-northeast there is a barrow, eighty-three feet long (from northeast to south west) and forty-six feet wide.

There are some other small remains of a similar character in Derbyshire, but I have not seen them myself, and doubt whether they are worth the trouble of a visit.

CHARAKA SAMHITA.

BY F. A. HASSLER, M.D., PH D., SANTA ANA, CAL. THE student of Hindu literature has before him an ever-widen

ing field of research. He must be prepared for glimpses and magnificent views of learning and wisdom which will astonish and delight him at every turn. The thoughts and the meth d of expression are different from those of other nations, and there is scarcely a subject, except, perhaps, electricity and steam, that has not been discussed by these ancient sages. The philosopher will find his theories, the anarchist his ideas, probed to the bottom, and thestudent of the supreme soul, high, noble thoughts, and even from this grand subject down to the every-day question of mistress and maid, we do not think of any matter that will not be found fully investigated in the pages of the Mahabharata.

So the physician of our day will find in the Charaka and other works of ancient India many views of health, disease, and remedies which he fondly imagined were jewels in the crown of modern science. When a young man wishes to study medicine, he may receive a little instruction from his pr ceptor, but places his chief reliance upon the teachings of some medical school from which he receives his diploma. This was not the custom in ancient India. There were no colleges. Every student became a part of his preceptor's household, was lodged and fed by him, and beyond a fer light services was not asked for any return. It is plain that such teachers could not instruct all their scholars by word of mouth. This accounts for the immense number of medical works of ancient India.

We cannot tell the age of the Charaka, it is based upon a work of Agniveca, which carries us back to almost mythical times. The very name of this supposed author sounds like the mystery of long past ages, for it may be translated "the dwelling-place of fire." Ten years of study of the Mahabharata has led me to quite certain conclusions as to the time when that great work was written, and I should say that the style, of the first part at least, of the Charaka corresponds with that portion of the Mahabharata which I think was written about the sixth century before Christ, or, in other words, about the time of the rise of Buddhism. Whatever its age may be this we know, it is exceedingly ancient. It is mentioned by Avicenna, Rhazes, and others, and is suppo-ed to have been translated by the early Persian and Arabian writers on medicine But we forget its age when we read its pages. The work is immense. An English translation, now being published by Doctor Kiviratna, the learned editor of several Sanscrit works and of a medical journal in Bengali, will probably cover from fourteen to fifteen hundred royal octavo pages. But it is not its size to which I wish to call attention, it is the wisdom and learning found in it that make it so valuable and interesting.

In a short article like this I cannot expect to do more than give the reader a glimpse of the work and a quotation here and there. We are told that in the earliest times some fifty-odd learned men assembled to study the science of life and the causes of disease; in fact, it was a medical convention similar to those of our day. The first conclusion they arrived at was that "Freedom from disease is the excellent root of religion, profit, pleasure, and salvation. Diseases are depredators thereof, as also of happy life. This, therefore, is a great enemy of men that hath appeared.

What shall be the means of checking them? Having said this, they betook themselves to meditation."

They did not discuss questions of life and health only, but moral and religious subjects also, and their effect upon life in general. The wind, or breath, disorders of the billiary system and phlegm, or improper secretions, seem to have been fully recognized as causes of bodily diseases, while passion and darkness of mind brought about mental disorders. Long lists of drugs and directions for their proper use are given, and there is abundant evidence that the properties of vaccine matter were well known. We are told that "He who knows how to apply these in disorders is conversant with the science of medicine." And listen to the following in regard to drugs and those who use them: "He who is acquainted with their applications according to considerations of time and place, after having observed their effects on individual patients, should be known as the best of physicians. An unknown drug is like poison, or weapon, or fire, or thunder, while a known drug is like nectar. Drugs unknown by name, appearance, and properties, or misapplied even if known, produce mischief. Well applied, a virulent poison, eren, may become an excellent medicine, while a medicine misapplied becomes a virulent poison. Only a physician who is possessed of memory, who is conversant with causes and applications of drugs, who has his passions under control, and who has quickness of decision, should, by the application of drugs, treat diseases."

Thirty-two kinds of powders and plasters and six hundred purgatives are next described, after which a chapter on food and its proper use gives us as good advice as is to be found in any treatise published in this learned nineteenth century. Great stress is laid upon the proper care of the teeth, and a list of plants is given from which brushes can be made, there not being manufactories of such articles as there are now.

"As the chief officer of a city protects his city, as the charioteer protects his chariot, after the same manner should the intelligent man be attentive to everything that should be done for the benefit of his own body." Therefore, bodily, mental, and, if we may so call it, religious hygiene is discussed at length, and many excellent rules given.

The question of the duality of the mind and of its connection with th understanding and the soul leads us into all the intricate mazes of Hindu philosophy, but are bere discussed in such a lucid manner that one is not bewildered and can easily follow the line of thought with pleasure and profit.

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The objects of the mind are ideas. Here, again, the proper, excessive, scant, and injudicious correlation of the mind with its objects, or of the mental understanding with its objects, becomes the cause of the normal or abnormal condition of oneself." In other words, a man is sane or insane according to the proper or improper agreement of the mind and its ideas, the ideas the understanding conceives; and, therefore, "One should act in such a way as to preserve one's normal condition, in order that one's untroubled senses and mind might continue in an untroubled state; that is to say, by keeping oneself in touch with such objects of the senses as are productive of beneficial results; by properly achieving such acts as deserve to be achieved (and abstaining from such acts as should be abstained from), repeatedly ascertaining everything by a judicious employment of the understanding; and. lastly, by resorting to practices that are opposed to the virtues of the place of habitation, season of time, and one's own particular nature or disposition (as dependant upon a preponderance of this or that attribute or ingredient). Hence all persons desirous of achieving their own good should always adopt with heedfulness the practices of the good."

Selfishness was never a cause of happiness, and we are told one can never be happy by taking or enjoying anything alone without dividing it with others." And this advice is good in every age of the world — “one should not trust everybody, nor should one mistrust everybody."

Hindu works teach that everyone should have complete mastery of his body and his senses, hence we frequently come across such a sentence as this: "One should not suffer oneself to be overcome by one's senses."

A very interesting chapter is that which treats of "The Aggre

gate of Four," that is, “the physician, nurse, drugs, and patient.” Each is considered and as good advice as can be found given for the guidance of three of the aggregate. One thing, the first of the four, is taught which it were well to remember in our day; that is, that time must be considered in the treatment of all diseases, and one must not try to force a cure.

It would take more time and space than are at our disposal for us to consider all of even the four parts of the Charaka that have been published so far, but if any of our readers are interested, we would be glad to give them any information in regard to the work or the other publications of the learned editor of this great monument of ancient Hindu wisdom and leaaning.

A NEW THEORY OF LIGHT SENSATION 1

BY CHRISTINE LADD FRANKLIN, JOHNS HOPKINS UNIVERSITY,
BALTIMORE, Md.

THE reasons which make it impossible for most people to accept either the Hering or the Young-Helmholtz theories of light sensation are familiar to every one. The following are the most important of them:

The Young-Helmholtz theory requires us to believe: (a) something which is strongly contradicted by consciousness, viz., that the sensation white is nothing but an even mixture of red-greenblue sensations; (b) something which has a strong antecedent improbability against it, viz., that under certain definite circumstances (e. g., for very excentric parts of the retina and for the totally color-blind) all three color-sensations are produced in exactly their original integrity, but yet that they are never produced in any other than that even mixture which gives us the sensation of white; (c) something which is quantitatively quite impossible, viz., that after-images, which are frequently very brilliant, are due to nothing but what is left over in the self-light of the retina after part of it has been exhausted by fatigue, although we have otherwise every reason to think that the whole of the self-light is excessively faint

The theory of Hering avoids all of these difficulties of the Young-Helmholtz theory, but at the cost of introducing others which are equally disagreeable; it sins against the first principles of the physiologist by requiring us to think that the process of building up highly organized animal tissue is useful in giving us knowledge of the externa' world instead of supposing that it takes place (as in every other instance known to us) simply for the sake of its future useful tearing down; it necessarily brings with it a quite hopeless confusion between our ideas of the brightness and the relative whiteness of a given sensation (as is proved by the fact that it enables Hering to rediscover, under the name of the specific brightness of the different colors, a phenomenon which has long been perfectly well known as the Purkinje phenomenon); the theory is contradicted (at least the present conception of it) by the following fact-the white made out of red and green is not the same thing as the white made out of blue and yellow; for if (being mixed on the color-wheel) these two whites are made equally bright at an ordinary intensity, they will be found to be of very different brightness when the illumination is made very faint.

Nevertheless, the theory of Hering would have to be accepted if it were the only possible way of escape from the difficulties of the Young-Helmholtz theory. But these difficulties may be met by a theory which has the following for its principal assumptions.

In its earliest stage of development vision consisted of nothing but a sensation of grey (if we use the word grey to cover the whole series black-grey-white). This sensation of grey was brought about by the action upon the nerve-ends of a certain chemical substance set free in the retina under the influence of light. In the course of development of the visual sense the molecule to be chemically decomposed became so differentiated as to be capable of losing only a part of its exciting substance at once; three chemical constituents of the exciter of the grey-sensation can therefore now be present separately (under the influence

1 Abstract from the Proceedings of the International Congress of Experimental Psychology, London, 1892.

of three different parts of the spectrum respectively), and they severally cause the sensations of red, green and blue. But when all three of these substances are present at once they recombine to produce the exciter of the grey sensation, and thus it happens that the objective mixing of three colors, in proper proportions, gives a sensation of no color at all, but only grey.

This theory is found, upon working it out in detail, to avoid the difficulties of the theories of Helmholtz and of Hering.

Its assumption of a separate chemical process for the production of the sensation of grey gives it the same great advantage over the Young-Helmholtz theory that is possessed by the theory of Hering; it enables it, namely, to account for the remarkable fact that the sensation of grey exists unaccompanied by any sensation whatever of color under the five following sets of circumstances when the portion of the retina affected is very small, when it is very far from the fovea, when the illumination is very faint, when it is very intense, and when the retina is that of a person who is totally color-blind. This advantage my theory attains by the perfectly natural and simple assumption of a partial decomposition of chemical molecules; that of Hering requires us to suppose that sensations so closely related as that of red and green are the accompaniments of chemical processes so dissimilar as the building up and the tearing down of photo-chemical substances, and farther that two complementary colors call forth photo chemical processes which destroy each other, instead of combining to produce the process which underlies the sensation of grey.

Of the first four of the above enumerated cases the explanation will readily suggest itself; in the case of the totally color-blind it is simply that that differentiation of the primitive molecules by which they have become capable of losing only a part of their exiting substance at one time has not taken place; the condition, in other words, is a condition of atavism. In partial color-blindness and in the intermediate zones of the retina in normal vision the only colors perceived are yellow and blue. This would indicate that the substance which in its primitive condition excites the sensation of grey becomes in the first place differentiated into two substances, the exciters of yellow and blue respectively, and that at a later stage of development the exciter of the sensation of yellow becomes again separated into two substances which produce respectively the sensations of red and of green. In this way the unitary (non-mixed) character of the sensation yellow is accounted for by a three-color theory as completely as by a four-color theory. A three-color theory is rendered a necessity by the fact that it alone is reconcilable with the results of König's experiments for the determination of the color-equations of color-blind and of normal eyes,' experiments which far exceed in accuracy any which have yet been made in color-vision, but which, owing to the intricate character of the theoretical deductions made from them, have not hitherto been allowed their due weight in the estimation of color theories.

The explanation which the theory of Hering gives of afterimages and of simultaneous contrast are not explanations at all, but merely translations of the facts into the language of his theory. My theory is able to deal with them more satisfactorily; when red light, say, has been acting upon the retina for some time, many of the photo-chemical molecules have lost that one of their constituents which is the exciter of the red sensation; but in this mutilated condition they are exceedingly unstable, and their other two constituents (the exciters of the sensations of blue and of green) are gradually set free; the effect of this is that, while the eyes are still open, a blue-green sensation is added to the red sensation with the result of making it gradually fade out into white, and, if the eyes are closed, the cause of the blue-green sensation persists until all the molecules affected are totally decomposed. Thus the actual course of the sensation produced by lcoking at a red object,—its gradual fading out, in case of careful fixation, and the appearance of the complementary color if the illumination is diminished or if the eyes are closed, -is exactly what the original assumption of a partial decomposition of molecules would require us to predict. The well-known extreme rapidity of the circulation in the retina would make it im1A King und C. Dieterici. Sitzungberichte der Berl. Akad.vʊm 29 Jul!, 1886.

possible that the partly decomposed molecules just referred to should remain within the boundaries of the portion of the retina in which they are first produced; and their completed decomposition after they have passed beyond these boundaries is the cause of the complementary color-sensation which we call simultaneous contrast. The spreading of the actual color which succeeds it would then be accounted for, as Helmholtz suggests, by a diffusion of the colored light in the various media of the eye.

No effort has hitherto been made to explain a very remarkable feature in the structure of the retina, -the fact that the retinal elements are of two different kinds, which we distinguish as rods and cones. But this structure becomes quite what one might expect, if we suppose that the rods contain the undeveloped molecules which give us the sensation of grey only, while the cones contain the color molecules, which cause sensations of grey and of color both. The distribution of the rods and cones corresponds exactly with the distribution of sensitiveness to just perceptible light and color excitations as determined by the very careful experiments of Eugen Fick.'

Two other theories of light sensation have been proposed besides the one which I have here outlined, either one of which meets the requirements of a possible theory far better than that of Hering or of Helmholtz; they are those of Göller and Donders. The former is a physical theory. That of Donders is a chemical theory, and very similar to the one which I here propose. Every chemical theory supposes a tearing down of highly complex molecules; Donders's theory supposes, in addition, that the tearing down in question can take place in two successive stages. But Donders's theory is necessarily a four-cǝlo r theory; and Donders himself, although the experiments of König above referred to had not at that time been made, was so strongly convinced of the necessity of a three-color theory for the explanation of some of the facts of color-vision that he supplemented his four-process theory in the retina with a threeprocess theory in the higher centres. The desirableness, therefore, of devising a partial decomposition of molecules of such a nature that the fundamental color-processes assumed can be three in number instead of four is apparent.

But the theory of Donders is open to a still graver objection. The molecules assumed by him must, in order to be capable of four different semi-dissociations, consist of at least eight different atoms or groups of atoms. The red green dissociations and the yellow-blue dissociations we may then represent symbolically by these two diagrams respectively:

But it will be observed that the two completed dissociations end by having set free different combinations; in the one case 1 is combined with 2 and in the other case 1 is combined with 8, etc. If, now, the partial dissociations are so unlike as to cause sensations of yellow and blue (or of red and green) it is not probable that completed dissociations which end in setting free different chemical combinations should produce the same sensation, grey. The difficulty introduced by Donders's theory is therefore (as in the case of Hering's theory) as great as the difficulty sought to be removed. It is the desire to secure the advantages of a partial dissociation theory, without the disadvantages of the theory of Donders, that has led me to devise a partial dissociation of molecules of a different kind. The theory will be found more explicitly set forth in the next number of the Zeitschrift für Psychologie.

2 Studien über Licht und Farbenempfindung. XLIV., s. 441, 1888.

Pflüger's Archiv, Bd.

3 Die Analyse der Lichtwellen durch das Auge. Du Bois-Reymond's Archiv, 1889.

4 Noch einmal die Farben-systeme. Grä e's Archiv für Ophthalmologie, Bd. 30 (1), 1884.