48o 



NATURE 



sation corresponds to blue and yellow, the blue rays ex- 

 citing it in one direction and the yellow rays in the other. 

 The other source corresponds to red and green, and is 

 excited in like manner. It will at once be seen with what 

 admirable simplicity this will explain colour-blindness, 

 avoiding the violence done to the evidence by the Young- 

 Helmholtz doctrine. Normal-eyed persons possess both 

 sources of sensation ; colour-blind persons possess only 

 one. The usual case is when the red-green source is 

 absent, the patient seeing only blue and yellow ; but the 

 other defect is possible, giving blindness to blue and 

 yellow, and vision only of red and green ; and Dr. Stilling, 

 who strongly espouses the theory, states that rare examples 

 of this have been found. If both sources of sensation 

 are absent, the patient sees only light and shade, and 

 this case also is said to have been practically known. 



It is a pity Dr. Jeffries has omitted to mention this 

 theory, which, if it should be substantiated by further 

 inquiry,' bids fair to be a most valuable contribution to 

 our knowledge. In the meantime the phenomena of 

 colour-blindness, from the important bearing they have 

 on the nature of colour-perception generally, require much 

 further careful investigation. 



William Pole 



OUR BOOK SHELF 



Elementary Lessons on Sound. By Dr. W. H. Stone, 

 Lecturer on Physics at St. Thomas's Hospital. (London : 

 Macmillan and Co., 1879.) 



Since the publication, some five and twenty years ago, of 

 Helmholtz's great work on musical acoustics, the study of 

 the nature of sound has become popular. The ordinary 

 phenomena of hearing must interest every one ; but it is 

 to the thoughtful student of music that the subject pre- 

 sents its chief attractions. We cannot imagine any intel- 

 ligent musician who will not be desirous to know some- 

 thing of the foundation of the wonderful fabric he has to 

 deal with, and to learn how the principles of science bear 

 on the practice of the art. 



It is well, therefore, that Messrs. Macmillan have in- 

 cluded among their School Class Books one which gives, 

 in a very small compass, a large amount of information 

 as to the laws and phenomena of sound. The author has 

 not only extracted the essence of what is contained in 

 bulky and expensive treatises, sometimes in foreign lan- 

 guages, but he has also given much additional information 

 from memoirs and transactions of scientific societies out 

 of the reach of the ordinary public. 



The application of acoustics to musical instruments is 

 a useful addition, the subject being one which the author 

 has made specially his own. He has also stated some 

 of the simplest facts of the connection between acoustical 

 phenomena and the structure of music ; but this is too 

 wide a subject, and involves far too complicated con- 

 siderations to be fully dealt with in an elementary work of 

 this kind. 



We notice a few trifling errors, as, for example, on page 

 3, the monochord can hardly be said to be " named after " 

 Pythagoras ; and Tartini's terzo siiono was intended by 

 him rather as a guide to correct double-stopping than 

 "tuning." On page 11, line 7, the expression "first 

 partial" is probably meant to be " first overtone." On 

 page 76 a pretty contrivance, by Mr. Francis Gallon, is 

 ascribed to Capt. Douglas Galton. These things are, 

 however, of little consequence. 



* It may be mentioned that one of the main points in the theory has lately 

 received unexpected and powerful support from the brilliant discoveries of 

 Bell and Kiihne in regard to the physiology of the retina. 



LETTERS TO THE EDITOR 



[ 7'he Editor does not hold himself responsille for opinions expressed 

 by his correspondents. Neither can he undertake to return, or 

 to correspond with the writers of, rejected manuscripts. No 

 notice is taken of anonymous communications. 



[The Editor urgently requests correspondents to keep their letters as 

 short as possible. 1 he pressure on his space is so great that it 

 is impossible otherwise to ensure the appearance even of com- 

 munications containing interesting and novel facts.\ 



Local Colour- Variation in Lizards 



The interest which some notes by Messrs. Wallace and 

 Giglioli (published in Nature) have called forth with regard 

 to the local variation of colour in reptiles causes me to publish 

 these few lines. 



Since the year 1874 I have been carefully studying this subject, 

 and therefore wish to remove the generally prevaUing opinion 

 that no endeavours have yet been made to explain it. I have 

 not thought it necessary to write this before, thinking that my 

 works touching this subject were known to naturalists, or would 

 have become known through the mention Mr. Carpenter makes 

 of them. Such, however, is not the case. Neither English 

 nor Italian zoologists have taken any notice of the newer German 

 publications concerning the local variation of colour in lizards. 

 They content themselves with merely mentioning many new and 

 truly interesting instances of this variation, but leave unnoticed 

 all attempts made to obtain an explanation of the same. 



The first effort to explain this appearance was made by Mr. 

 Eimer in 1 872, at the time that the beautiful black and blue lizard 

 was discovered on the Faraglioni rocks, near Capri. Prof. 

 Eimer tries to explain this change of colour in the Laccrta 

 muralis (which is green both on the Continent and the Island of 

 Capri) by attributing it to an adaptation to the colour of the 

 Faraglioni rocks. However, as those rocks are not of a bluish- 

 black, but rather a yellowish-red colour, intermixed with a little 

 gray, and as, moreover, the lizards there have no enemies against 

 which they require protection, and therefore no adaptation is 

 necessary, I considered Prof. Elmer's explanation a failure, and 

 at the same time I tried to confirm by fresh facts my hypo- 

 thesis m.ide in 1874 (" Ueber die Entstehung der Farbenbei den 

 Eidechsen," Jena, 1874). This hypothesis, which, it is true, has 

 till now met with little approval, is as follows : — The skin of the 

 lizard has two layers of pigment. The black pigment, which 

 lies lowest, gets the power, under the concentrated influence of 

 the sun, to leave its motionless state, and is made to rise by the 

 contraction which the nerves exercise on the cells containing it, 

 and by forcing itself more or less upwards through the elements 

 of the pale layer of pigment, gives us the impression of diffejeut 

 colours. That change of colour which we are able to observe in 

 chameleons in a short space of time, under the condition of a 

 frequent change of light, takes place with lizards only in the 

 course of ages, embodying itself in manifold degrees of develop- 

 ment, and provided the animal does not change the locality, 

 remains as a distinguishing characteristic of the form. If, how- 

 ever, the lizard changes its locality, if it is isolated on a rock 

 or islet which has separated itself from the mainland, and is en- 

 tirely and constantly exposed to the rays of the sun, as must 

 naturally be the case on rocks which, like the Faraglioni or the 

 Island of Ayre, are void of all vegetation, in that case, I say, the 

 black layer of pigment is set in motion, and by constant suc- 

 cessive risings to the surface at last gains a definite superposition 

 over the yellow pigment, as has been the case with the black 

 FaragUoni and Lilfordi lizards. 



This phylogenetic development of colours can be traced (as I 

 have already mentioned in the year 1874) by the individual deve- I 

 lopment of colour in the lizard, but necessarily only under the ; 

 constant strong influence of the sun on young individuals. Dr. 

 Braun, in his work on the Lacerta lilfordi, informs us that the 

 young lizard of tlie Island of Ayre has exactly the same colour as 

 its typical form on the larger Balearic islands, and only turns 

 black in the course of its growth. 



Though we can only observe the turning black of these liza; 

 in the individual growth of the animal, we can obtain a return', 

 of the full•gro^vn animals to their original paler colours by a: 

 ficial means, that is, by preventing the rays of the sun fi 

 falling on them perpendicularly. By these means I completely 

 discoloured numbers of the Faraglioni lizards and the brown ones 

 from the island of Ponza. The former turned bluish-green, tlie 

 latter brownish-green. 



Before I pass on to an enumeration of the above-named trans- 



