These two latter cables have recently been completed, and the Shanghae-Possiette cable is now 'in course of submergence; the Hong-Kong-Shanghae cable was successfully laid last month. These lines give a total distance of over 3,978 nautical miles of submarine cable with Hooper's indiarubber insulation. The following observations as regards the electrical conditions of these cables as compared with well-known gutta-percha insulated cables is remarkable. The electrical tests of well-known cables with both the gutta-percha and the Hooper core are taken at the temperature of 75° Fahr., and in terms of British Association (B.A.) units, the standard measure now most generally adopted in England for comparison:

With such results, it is not to be wondered at that the relative speed of two cables of similar length and construction, the one employing a gutta-percha core and the other a Hooper core, should be found greatly in favour of the latter, in the proportion of 130 to 100; that is to say, in any given time the Hooper core, from its superior insulating properties, will transmit thirty per cent. more words than a gutta-percha core, a most important circumstance when it is considered that the earnings or dividend upon each cable is dependent upon the work it can perform in a given period. As regards the apparatus employed for transmitting the currents through submarine conductors, the "Wheatstone" automatic recording system is the most successful. By this apparatus an average speed of over thirty words a minute is regularly maintained upon the Danish-English cable, a distance of 363 nautical miles, exclusive of a further land circuit of over 140 miles, making a total distance of about 500 miles. This speed must be compared with that of seventeen words per minute, the highest result recorded over the same circuit by the most improved Morse system. From the esults of the "Wheatstone" apparatus working over this circuit since September 1868, it appears that to obtain maximum speed, the currents through a submarine cable require to be transmitted of equal duration, at equal intervals, in alternate directions, and the line discharged to earth between each successive reversal or current to neutralise the charge, all of which conditions are fulfilled in the "Wheatstone" Automatic Jacquard arrangement, which can only be compared to a loom weaving the currents into the line, the sequence of the currents representing the pattern on the cloth. This apparatus is now organised as the transmitting and recording register upon the vast system of submarine circuits belonging to the Great Northern Telegraph Company, and the extensions from Possiette Bay (RussianChinese frontier) to Nagasaki, Shanghae, and Hong-Kong. The subject of high speed transmission through insulated conductors, both by land and sea, is one which demands

special attention, now that the telegraph is daily encroaching upon the postal service, a service in which both speed and accuracy are more than ever demanded by the public. NATH. J. HOLMES

PFLÜGER'S NERVE ENDINGS IN GLANDS

I

N his "Archiv für die Gesammte Physiologie" (Bonn, 1871), E. Pflüger gives a short and summary answer to those many observers who have thrown doubt on the accuracy of his remarkable discoveries as to the continuity of nerves with the secreting cells of the salivary glands and liver. Pflüger's opponents in this matter have been Mayer, Hering, Krause, Henle, and Schweigger-Seidel. The objections which have been made are divided by him into three heads. 1st. It was said that the nerves he had seen were capillary vessels. 2nd. That they were threads of mucus. 3rd. They were disintegrated fat. These objections are successively shown to be groundless, and Pflüger stoutly maintains his original position. What is far more important in this short paper than these answers to objections is that the professor at length publishes an account of some of his methods as to which he has so long left every one in the dark. They are certain to be interesting to some of our readers. Salivary glands. A fresh submaxillary gland from the ox must be taken, and very fine sections made; these must be at once teased out in perosmic acid sp. gr. 1003, and covered with a thin glass in a shallow cell. A great many preparations should be made, and the best picked out. They will be sufficiently stained in 24 hours. As the water dries up it may be replaced by glycerine. Liver. A great number of very fine sections must be made from the fresh liver of a dog or pig. These sections must be placed 10 or 12 together in watch-glasses filled with Beale's carmine solution, and thus kept in a moist chamber 14 days. The sections must then be taken out, washed one by one in a drop of perosmic acid, sp. gr. 1003, transported to a fresh drop of the same on a slide, and carefully teased out, covered, and examined.

NOTES

ST. BARTHOLOMEW'S HOSPITAL has, we learn from the

British Medical Journal, sustained a great loss in the resignation by Mr. Paget of his active duties as Surgeon to the Hospital. Mr. Paget will, of course, receive the appointment of Consulting Surgeon to the Institution which he has served long and faithfully, and on which he has conferred lustre.

THE following excursions have been arranged by the Geologists' Association to take place in May :-To Oxford on Friday, 12th May. On arriving at Oxford the New University Museum will be visited. Subsequently the party, accompanied by the President, Prof. Phillips, and Prof. Morris, will walk to Shotover Hill, where the Middle and Upper Oolites are well exposed. To Grays, Essex, on Saturday, 20th May. Exposures of the Mammaliferous beds of the Thames Valley, and afterwards sections of the Upper Chalk will be visited, under the guidance of Prof. Morris. A four days' excursion to Yeovil, Weymouth, and Portland is proposed for Whitsuntide. Particulars of arrangements will be duly announced.

THE Edinburgh Naturalists' Field Club, which has since its formation carried on active operation only from April to July inclusive, held its adjourned annual meeting and conversazione on Saturday, the 22nd April, when Mr. Robert Scot-Skirving, the president, delivered an introductory address, enlarging mainly on entomology as a fit summer field study. The business meeting was held in November last, when, in addition to the

present president and committee, Prof. Liston was elected viceresident, and Mr. Andrew Taylor honorary secretary and

treasurer.

THE following is the programme of the lectures on the Experimental and Natural Sciences in Trinity Term, in Trinity College, Dublin. Mineralogy, 11 A. M., on Mondays, Wednesdays, and Fridays. Demonstrations in Organic Chemistry, 12, Mondays, Wednesdays, and Fridays. Magnetism, 2 P.M.,

Mondays, Wednesdays, and Fridays. Comparative Anatomy, II A.M., Mondays, Wednesdays, and Fridays. Demonstrations in Botany, II A. M., Tuesdays, Thursdays, and Saturdays. Applied Geology, I P.M., Tuesdays, Thursdays, and Saturdays.

THE grace for allowing French and German as an alternative for Greek, was submitted to the Senate of the University of Cambridge on Thursday last, and rejected by a majority of three only. The subject will doubtless be reopened, and probably some slight modification of the original scheme will ultimately be accepted.

THE following gentlemen have been elected, by the Senate of the University of London, Examiners in Science and Medicine for the ensuing year :-Logic and Moral Philosophy: Prof. G. Croom Robertson and Rev. John Venn. Political Economy: Prof. W. Stanley Jevons and Prof. T. E. Cliff Leslie. Mathematics and Natural Philosophy: Prof. H. J. S. Smith, F.R.S., and Prof. Sylvester, F. R. S. Experimental Philosophy: Prof. W. G. Adams and Prof. G. Carey Foster, F. R. S. Chemistry: Henry Debus, F.R.S., and Prof. Odling, F.R.S. Botany and Vegetable Physiology: Dr. J. D. Hooker, F.R.S., and Dr. Thos. Thomson, F.R.S. Geology and Paleontology: Prof. Duncan, F.R.S., and Prof. Morris. Practice of Medicine: John S. Bristowe, M.D., and Prof. J. Russell Reynolds, M.D., F.R.S. Surgery: Prof. John Birkett and Prof. John Marshall, F.R.S. Anatomy: Prof. Geo. Viner Ellis and Prof. John Wood. Physiology, Comparative Anatomy, and Zoology: Prof. M. Foster, M. D., and H. Power. Obstetric Medicine: Rob. Barnes, M.D., and Prof. W. M. Graily Hewitt, M.D. Materia Medica and Pharmaceutical Chemistry: Thos. R. Fraser, M.D., and Prof. A. Baring Garrod, M.D., F.R.S. Forensic Medicine : J. Headlam Greenhow, M.D., F.R.S., and Thos. Stevenson, M.D.

MR. C. T. CLOUGH, of Rugby School, has been elected to an exhibition at St. John's College, Cambridge, of 50l. per annum, for proficiency in Natural Science. There were ten candidates.

SCIENCE appears to have penetrated even into the recesses of Christ's Hospital. Since October 1869, there has been a Chemistry class of about fifty boys, in connection with St. Bartholomew's 1ospital. The work done is both practical and theoretical; at the first, Dr. Matthiessen was the lecturer, and at his lamented death, D. H. E. Armstrong. Since Christmas the class has been der the care of Dr. W. T. Russell, F. C.S. For some weeks past, Prof. Tennant has been lecturing on Mineralogy, and next week commences on Geology. This class is very well attended. There has been established a permanent class for Natural ilosophy, under the care of Mr. James Noon, B. A. We believe ao that those boys who are intended for the Navy are insucted in theoretical and practical Astronomy. There have en wishes expressed for a Museum, and numerous speciens are constantly brought to Prof. Tennant for informan. It is much to be wished also that some sort of a Natural 1 story Society might be established, notwithstanding the city-site of the Hospital.

WE continue to receive intelligence from the French Academy, and are in a position to give the full list of members who were

present at the sitting on the 21st April, eighteen in number, viz., three astronomers, Yvon Villareau, Mathieu, and Langier ; one mathematiciau, Chasles; one physicist, Jamin, three chemists, Chevreul, Payer, Peligot; one mechanician, Ameral Paris; and the others medical men or naturalists, Milne-Edwards, Blanchard, Robin, Trécul, Bienaymè, Duchartre, and Quatrefages. M. Egger, of the Academy of Inscriptions, sat with his colleagues, and M. Simon Newcomb, the American astronomer, sat at the place allotted to foreign learned men.

THE seventh part of the illustrated work on the butterflies of North America, by Mr. Wm. H. Edwards, has just been published, containing numerous well-engraved and coloured plates of butterflies.

THE Commune has its own balloons, twelve in number, but they are kept apart for the private use of members when the final exit shall take place. One was sent up into the air, as it was said in the political newspapers for carrying away the masonic proclamation, but it was a little one without aëronaut.

THE Gardener's Chronicle for last Saturday prints an interesting letter from Dr. Hooker, dated Tetuan, April 12. In the journey from Tangier to Tetuan, Dr. Hooker notices that the general features of the flora of the low grounds and moderate hills in that part of North Morocco coincide with those of Southwestern Spain. Whole tracts are covered with masses of broom, so that the hills precisely resemble those of Scotland or Jersey. The previous day a guard had been obtained in order to ascend Beni-Hosmar, which mountain had only been visited previously by one botanist, Mr. Webb, some forty years since. The party ascended to 3,500 feet, and obtained a superb view across the Mediterranean to the Spanish coast, and south to the snowy crest of Beni-Hassan. It is a splendid rugged mass of limestone peaks, separated by very steep narrow-floored valleys, the flanks of which are crested with rifted white precipices. The whole is clothed with stunted shrubs up to 3,000 feet. They found some rare, and some probably new plants, but at a height of 3,400 feet no signs of a sub-alpine flora. The party did not succeed in reaching the summit.

THE principal object of interest at the soirée of the Linnean Society on the 26th ult., was again Mr. Wilson Saunders's collection of mimetic plants, which was even more remarkable than last year. The following is a list of the pairs exhibited :

A SERIES of scientific lectures, in connection with the School of Science and Art, has been for some time in contemplation at Taunton. It has recently commenced with a course of botanical lectures, by the Rev. W. Tuckwell, headmaster of the College School, which attract a large and diligent audience, consisting both of artisans and amateurs.

Ar the annual dinner of the Institution of Civil Engineers held on April 22, Prof. Huxley, in responding to the toast of the learned societies in this country, gave the company some very sound advice as to the duty of the body of civil engineers in enforcing upon the public mind the truth that there can be no technical education of any value or soundness which is not based on a thorough preliminary training in abstract or theoretical practical science.

MR. JOHN GIBBS, of the Essex and Chelmsford Museum, publishes, at the price of a shilling, "A First Catechism of Botany," which has received the sanction of the Committee of Selection for the International Exhibition. Although the catechismal form always seems to us a needlessly cumbrous and circumlocutory one, for those who think otherwise a large amount of useful elementary information will be found in this little publi

cation.

WE learn from Trübner's American and Oriental Literary Record that the American Ethnological Society was permanently reorganised under the name of “The Anthropological Institute of New York," on the 9th of March, and the following officers were elected :-E. Geo. Squier, president; J. C. Nott and Geo. Gibbs, vice-presidents; J. G, Shea, J. K. Merrill, E. H. Davis, C. C. Jones, jun., and W. H. Thomson, executive committee; A. J. Cotteral, treasurer; Charles Raw, foreign corresponding secretary; H. T. Drowne, domestic corresponding secretary; H. R. Stiles, recording secretary; and Geo. H. Moore, custodian. The objects of the institute are declared to be:-1. The study of man in all his varieties, and under all his aspects and relations. 2. Its special object the study of the history, conditions, and relations of the aboriginal inhabitants of America, and the phenomena resulting from the contact of the various races and families of men on this continent, before and since the discovery.

3. The physical characteristics, religious conceptions, and systems of men ; their mythology and traditions; their social, civil, and political organisations and institutions; their language, literature, arts, and monuments; their mode of life and their customs are specifically within the objects of the Institute. 4. The collection of manuscripts, books, and relics illustrating these several subjects; the stimulation and encouragement of inquiry and research, particularly in unexplored American fields; and by means of such publications as may be deemed proper, to utilise the results of its investigations and efforts for the benefit of science and of mankind. 5. It recognises the widest range of discussion, and a complete tolerance of individual opinions on all subjects within the scope of the Institute's objects. The Institute proposes to publish a series of Memoirs, and a Journal.

THE attention of astronomers throughout the world is directed toward the approaching transit of Venus, to occur on the 18th of December, 1874, and it is hoped, we learn from Harper's Weekly, that the United States Congress, with the same liberality that induced it to make an appropriation for the observation of the solar eclipse of December last, and for the polar exploration under Captain Hall, will also, at the proper time, advance the funds necessary for the research in this case. Professor Hall, of the Washington Observatory, in a late communication to the Journal of Science, expresses the hope that a concert of action will be settled upon by American astronomers, in order that they may not be behind their European confrères in the attempt to secure satisfactory results. A committee has been appointed by

the National Academy of Sciences to take into consideration a general plan of operations, and it is expected that a report will be made on the subject at the approaching meeting in Washington city.

THIE annual report of Professor Cooke, State Geologist of New Jersey, for 1870, has just been published; and although less in bulk than some of its predecessors, it contains some important information in regard to fertilisers used in the State, the marshes and tracts of land subject to protracted freshets, the soils, the iron and zinc ores, and other miscellaneous topics. The subject of drainage has attracted Professor Cooke's especial attention, on account of the vast tracts of land in the eastern portion of the State now either regularly overflowed at certain periods of the tide, or liable to freshets or inundations. In order more properly to qualify himself for this inquiry, Professor Cooke paid an extended visit to the drained lands of Holland and England, the results of which he presents in his report.

M. LONGET, the celebrated physiologist, member of the French Institute and of the French Academy of Medicine, died at the age of sixty-eight, at Bordeaux, a few days since. M. Longet is the author of works on the nervous system, which explain many of his own discoveries. His death was sudden, and was referred by his friends to the horror with which he was stricken when hearing the sad news from Paris.

In the forthcoming number of the American Journal of Science will be found an article, by Professor Marsh, upon some new serpents of the Tertiary deposits of Wyoming. It will be remembered that in a previous notice of Professor Marsh's discoveries in the Rocky Mountains, we called attention to the difference observed by him between the contents of the Tertiary beds in the

vicinity of Fort Bridger and those of the Mauvaises Terres of the Upper Missouri, the former being especially characterised, as compared with the latter, by the presence of reptiles in great variety. Among these are many terrestrial species, including several kinds of land lizards; and among the forms generally serpents appear to be quite predominant. Of the latter, Professor Marsh has already determined the existence of five new species, belonging to three new genera; and others will probably be yet brought to light.

AT the present day, when the columns of our newspapers teem with advertisements of various preparations for promoting the growth or changing the colour of the hair, the following account of the results of the use of a preparation of boxwood for that purpose may be of interest. Boxwood, according to the old herbalists, was used from a remote period to render the hair auburn; and we are told by Phillips that a young woman in Lower Silesia, whose hair had fallen off after a severe attack of dysentery, was advised to wash her head with a decoction of boxwood, in order to induce it to grow again. This she did; and "hair of a chestnut colour grew on her head, as she was told it would do; but, having used no precaution to secure her face and neck from the lotion, they became covered with red hair to such a degree that she seemed but little different from an ape or a monkey!"

MR. JAMES BOYD, of Panama, published some time ago in the Panama Star and Herald, under the head of "The Migration of Butterflies across the Isthmus," "" an account of the phenomenon of the migration in one direction of the Urania Leilus. This being republished in England has led to some correspondence with Mr. Boyd, particularly from a naturalist resident at Liverpool. This gentleman states that in January 1845, he observed the same habit of the Urania in the Island of Caripi, one of those near Para, in the Brazils. From an early hour in the morning until nearly dark these insects passed along the shore in amazing numbers, but most numerously in the evening. It was very seldom that one was seen in the opposite

direction. The main course was from west to east. He also saw it at Pernambuco, at Rio Janeiro, and in the Southern States of America, but nowhere so abundant as on the Amazons. The Urania is scarcely a butterfly; but between the day and night butterflies, something between a skipper and a hawk moth. By Latreille they were called Hespero-Sphyngidæ. The larvæ and pupæ are supposed not to have been adequately examined. The Liverpool naturalist could not identify them, and as yet they have not been able to find them at Panama. In Central America the Urania is found as far north as Guatemala. Mr. Darwin observed a butterfly of similar habits, the Papilio feronia, which frequents orange groves.

THE

THE ROYAL SOCIETY'S LIST FOR 1871 HE following fifteen have been selected by the Council of the Royal Society out of the fifty candidates, and recommended to the Fellows for election:- William Henry Besant, M.A., Mathematical Lecturer at St. John's College; Senior Wrangler and First Smith's Prizeman in 1850, Moderator in 1856, Examiner in the University of London from 1860 to 1865; author of Treatises on "Hydromechanics and the Theory of Sound," 2nd ed. 1867; "Elementary Hydrostatics," 2nd ed. 1867; "Geometrical Conic Sections," 1869; "Roulettes and Glissettes," 1870. William Budd, M.D. (Edin.), physician, author of various medical papers, especially relating to contagious diseases. George W. Callender, F.R.C.S., lecturer on Anatomy at St. Bartholomew's Hospital School, and Assistant Surgeon to Bartholomew's Hospital; author of Anatomical papers. William Carruthers, F.L.S., F.G.S., keeper of the Botanical Department, British Museum; author of "Fossil Cycadean Stems from the Secondary Rocks of Britain" "On the Structure and Affinities of Sigillaria and Allied Genera;" "The Cryptogamic Forests of the Coal Period;" "On the Structure of the Stems of the Arborescent Lycopodiaceae of the Coal-measures;" "Revision of the British Graptolites," &c. Robert Etheridge, F.R.S.E., F.G.S., Paleontologist to H.M. Geological Survey of Great Britain; Demonstrator on Palæontology, Royal School of Mines; author of numerous geological papers. Frederick Guthrie, B.A., F.R.S.E., F.C.S., Professor of Physics in the Royal School of Mines; author of various papers on Chemistry and Physics. Captain John Herschel, R.E., of the Great Trigonometrical Survey of India. Captain Alexander Moncrieff, Militia Artillery, C.E., inventor of the Moncrieff gun-carriage, and author of the Moncrieff system of defence. Richard Quain, M.D. (Lond.), Fellow and late Censor of the Royal College of Physicians; author of a paper "On Fatty Degeneration of the Heart," which has exerted a marked influence on certain branches of Pathological Science; and of numerous communications published in the Transactions of the Pathological Society, of which Society he was President (1869-70). Carl Schorlemmer, Senior Assistant in Owens College Laboratory, Manchester; author of a series of papers on the Constitution of the Paraffins, chiefly published in the Proceedings of the Society since 1862. Edward Thomas, Treas. R. A.Š., author of numerous papers on Indian Coins and Gems. Edward Burnet Tylor, author of "Researches into the Early History of Mankind;" "Primitive Culture ;" and various memoirs on Savages and their Customs. Cromwell Fleetwood Varley, Civil and Telegraphic Engineer, M.I.C.E.; Consulting Electrician to the Electric and International Telegraph Company, the Atlantic Telegraph Company, la Société du Cable Transatlantique Français; author of many inventions in connection with the Electric Telegraph. Viscount Walden, President of the Zoological Society of London; author of various papers on Ornithology. John Wood, F.R.C.S., Examiner in Anatomy at the University of London; author of a number of anatomical papers published in the Phil. Trans.

ON COLOUR VISION *

13

ALL vision is colour vision, for it is only by observing differences of colour that we distinguish the forms of objects. I include differences of brightness or shade among differences of colour. It was in the Royal Institution, about the beginning of this century, that Thomas Young made the first distinct announcement of that doctrine of the vision of colours which I propose to illustrate. We may state it thus :-We are capable of feeling three different colour-sensations. Light of different kinds excites these sensations in different proportions, and it is by the different combinations of these three primary sensations that all the varieties of visible colour are produced. In this statement there is one word on which we must fix our attention. That word is, Sensation. It seems almost a truism to say that colour is a sensation; and yet Young, by honestly recognising this elementary truth, established the first consistent theory of colour. So far as I know, Thomas Young was the first who, starting from the well-known fact that there are three primary colours, sought for the explanation of this fact, not in the nature of light, but in the constitution of man. Even of those who have written

on colour since the time of Young, some have supposed that they ought to study the properties of pigments, and others that knowledge of colour by examining something in external nature they ought to analyse the rays of light. They have sought for a -something out of ourselves.

Now, if the sensation which we call colour has any laws, it must be something in our own nature which determines the form of these laws; and I need not tell you that the only evidence we can obtain respecting ourselves is derived from consciousness.

The science of colour must therefore be regarded as essentially a mental science. It differs from the greater part of what is called mental science in the large use which it makes of the physical sciences, and in particular of optics and anatomy. But it gives evidence that it is a mental science by the numerous illustrations which it furnishes of various operations of the mind. dealing with physical science. In this place we always feel on firmer ground when we are I shall therefore begin by showing how we apply the discoveries of Newton to the manipulation of light, so as to give you an opportunity of feeling for yourselves the different sensations of colour.

Before the time of Newton, white light was supposed to be of all known things the purest. When light appears coloured, it was supposed to have become contaminated by coming into contact with gross bodies. We may still think white light the emblem of purity, though Newton has taught us that its purity does not consist in simplicity.

These

We now form the prismatic spectrum on the screen. are the simple colours of which white light is always made up. We can distinguish a great many hues in passing from the one end to the other; but it is when we employ powerful spectroscopes, or avail ourselves of the labours of those who have mapped out the spectrum, that we become aware of the immense multitude of different kinds of light, every one of which has been the object of special study. Every increase of the power of our instruments increases in the same proportion the number of lines visible in the spectrum.

All light, as Newton proved, is composed of these rays taken illuminated by white light, make a selection of these rays, and in different proportions. Objects which we call coloured when them. But if they receive only the pure rays of a single colour our eyes receive from them only a part of the light which falls on of the spectrum, they can appear only of that colour. If I place a disc containing alternate quadrants of red and green paper in the red rays, it appears all red, but the red quadrants brightest. If I place it in the green rays both papers appear green, but the red paper is now the darkest. This, then, is the optical expla nation of the colours of bodies when illuminated with white light They separate the white light into its component parts, absorbing some and scattering others.

Here are two transparent solutions. One appears yellow, it tains sulphate of copper. If I transmit the light of the electric contains bichromate of potash; the other appears blue, it conlamp through the two solutions at once, the spot on the screen appears green. By means of the spectrum we shall be able to explain this. The yellow solution cuts off the blue end of the spectrum, leaving only the red, orange, yellow, and green. The blue solution cuts off the red end, leaving only the green, blue, and violet. The only light which can get through both is the * Lecture delivered before the Royal Institution, March.24th.

green light, as you see. In the same way most blue and yellow paints, when mixed, appear green. The light which falls on the mixture is so beaten about between the yellow particles and the blue, that the only light which survives is the green. But yellow and blue light when mixed do not make green, as you will see if we allow them to fall on the same part of the screen together.

It is a striking illustration of our mental processes that many persons have not only gone on believing, on the evidence of the mixture of pigments, that blue and yellow make green, but that they have even persuaded themselves that they could detect the separate sensations of blueness and of yellowness in the sensation of green.

We have availed ourselves hitherto of the analysis of light by coloured substances. We must now return, still under the guidance of Newton, to the prismatic spectrum. Newton not only.

Untwisted all the shining robe of day,

but showed how to put it together again. We have here a pure spectrum, but instead of catching it on a screen, we allow it to pass through a lens large enough to receive all the coloured rays. These rays proceed, according to well-known principles in optics, to form an image of the prism on a screen placed at the proper distance. This image is formed by rays of all colours, and you see the result is white. But if I stop any of the coloured rays, the image is no longer white, but coloured; and if I only let through rays of one colour, the image of the prism appears of that colour.

I have here an arrangement of slits by which I can select one, two, or three portions of the light of the spectrum, and allow them to form an image of the prism while all the rest are stopped. This gives me a perfect command of the colours of the spectrum, and I can produce on the screen every possible shade of colour by adjusting the breadth and the position of the slits through which the light passes. I can also, by interposing a lens in the passage of the light, show you a magnified image of the slits, by which you will see the different kinds of light which compose the

mixture.

The colours are at present red, green, and blue, and the mixture of the three colours is, as you see, nearly white. Let us try the effect of mixing two of these colours. Ked and blue form a fine purple or crimson, green and blue form a sea-green or skyblue, red and green form a yellow.

Here again we have a fact not universally known. No painter, wishing to produce a fine yellow, mixes his red with his green. The result would be a very dirty drab colour. He is furnished by nature with brilliant yellow pigments, and he takes advantage of these. When he mixes red and green paint, the red light scattered by the red paint is robbed of nearly all its brightness by getting among particles of green, and the green light fares no better, for it is sure to fall in with particles of red paint. But when the pencil with which we paint is composed of the rays of light, the effect of two coats of colour is very different. The red and the green form a yellow of great splendour, which may be shown to be as intense as the purest yellow of the spectrum.

I have now arranged the slits to transmit the yellow of the spectrum. You see it is similar in colour to the yellow formed by mixing red and green. It differs from the mixture, however, in being strictly homogeneous in a physical point of view. The prism, as you see, does not divide it into two portions as it did the mixture. Let us now combine this yellow with the blue of the spectrum. The result is certainly not green; we may make it pink if our yellow is of a warm hue, but if we choose a greenish yellow we can produce a good white.

You have now seen the most remarkable of the combinations of colours-the others differ from them in degree, not in kind. I must now ask you to think no more of the physical arrangements by which you were enabled to see these colours, and to concentrate your attention upon the colours you saw, that is to say, on certain sensations of which you were conscious. We are here surrounded by difficulties of a kind which we do not meet with in purely physical inquiries. We can all feel these sensations, but none of us can describe them. They are not only private property, but they are incommunicable. We have names for the external objects which excite our sensations, but not for the sensations themselves.

When we look at a broad field of uniform colour, whether it is really simple or compound, we find that the sensation of colour appears to our consciousness as one and indivisible. We

cannot directly recognise the elementary sensations of which it is composed, as we can distinguish the component notes of a musical chord. A colour, therefore, must be regarded as a single thing, the quality of which is capable of variation.

To bring a quality within the grasp of exact science, we must conceive it as depending on the values of one or more variable quantities, and the first step in our scientific progress is to determine the number of these variables which are necessary and sufficient to determine the quality of a colour.. We do not require any elaborate experiments to prove that the quality of colour can vary in three and only in three independent ways.

One way of expressing this is by saying, with the painters, that colour may vary in hue, tint, and shade.

The finest example of a series of colours varying in hue, is the spectrum itself. A difference in hue may be illustrated by the difference between adjoining colours in the spectrum. The series of hues in the spectrum is not complete; for, in order to get purple hues, we must blend the red and the blue.

Tint may be defined as the degree of purity of a colour. Thus, bright yellow, buff, and cream-colour, form a series of colours of nearly the same hue, but varying in tint. The tints corresponding to any given hue form a series, beginning with the most pronounced colour, and ending with a perfectly neutral tint. Shade may be defined as the greater or less defect of illumination. If we begin with any tint of any hue, we can form a gradation from that colour to black, and this gradation is a series of shades of that colour. Thus we may say that brown is a dark shade of orange.

The quality of a colour may vary in three different and independent ways. We cannot conceive of any others. In fact, if we adjust one colour to another, so as to agree in hue, in tint, and in shade, the two colours are absolutely indistinguishable. There are therefore three, and only three, ways in which a colour can vary.

I have purposely avoided introducing at this stage of our inquiry anything which may be called a scientific experiment, in order to show that we may determine the number of quantities upon which the variation of colour depends by means of our ordinary experience alone.

Here is a point in this room: if I wish to specify its position, I may do so by giving the measurements of three distances-namely, the height above the floor, the distance from the wall behind me, and the distance from the wall at my left hand.

This is only one of many ways of stating the position of a point, but it is one of the most convenient. Now, colour also depends on three things. If we call these the intensities of the three primary colour sensations, and if we are able in any way to measure these three intensities, we may consider the colour as specified by these three measurements. Hence the specification of a colour agrees with the specification of a point in the room in depending on three measurements.

Let us go a step farther, and suppose the colour sensations measured on some scale of intensity, and a point found for which the three distances, or co-ordinates, contain the same number of feet as the sensations contain degrees of intensity. Then we may say, by a useful geometrical convention, that the colour is represented to our mathematical imagination by the point so found in the room; and if there are several colours, represented by several points, the chromatic relations of the colours will be represented by the geometrical relations of the points. This method of expressing the relations of colours is a great help to the imagination. You will find these relations of colours stated in an exceedingly clear manner in Mr. Benson's "Manual of Colour," one of the very few books on colour in which the statements are founded on legitimate experiments.

There is a still more convenient method of representing the relations of colours, by means of Young's triangle of colours. It is impossible to represent on a plane piece of paper every conceivable colour, to do this requires space of three dimensions. If, however, we consider only colours of the same shade, that is, colours in which the sum of the intensities of the three sensations is the same, then the variations in tint and in hue of all such colours may be represented by points on a plane. For this purpose we must draw a plane cutting off equal lengths from the three lines representing the primary sensations. The part of this plane within the space in which we have been distributing our colours will be an equilateral triangle. The three primary colours will be at the three angles, white or gray will be in the middle, the tint or degree of purity of any colour will be expressed by its distance from the middle point, and its hue