596 



NA TURE 



[April 24, 1902 



edges of the nebulse ; and the curved and inclined nature of the 

 lines, when compared with those in the comparison spectra, 

 indicate still more clearly relative movements in the nebulse 

 themselves. 



The Red Si-ot on JuriiER. — In the Astroiomischc 

 Na</trichlen (Bd. 158, No. 3786), Mr. .\. Stanley Williams 

 gives a discussion of his observations of the red spot on Jupiter 

 during 1901. The planet was badly placed owing to its great 

 southerly declination, but during the summer months the defini- 

 tion was exceptionally good. Tables are given of the times 

 occupied in the transit of the spot over the mean meridian of 

 the disc, measures being taken from both the middle and follow- 

 ing end of the marking. The mean rotation period thus 

 determined is gh. 55m. 40'92s., which is i '.^Ss. shorter than the 

 value deduced from the observations of the previous year. This 

 shortening of the length of the rotation period has also been 

 noted by several other observers. In appearance the spot was 

 very faint, especially at the preceding end. The following 

 extremity, however, was fairly dark ; a distinct though faint 

 reddish tinge was generally noticeable. 



Several reductions of measures of the length of the spot are 

 given as evidence in favour of Prof. G. W. Hough's statement 

 that the spot has not materially changed in size during the last 

 twenty years. 



THE MEANING OF THE WHITE UNDER 

 SIDES OF ANIMALS. 



pROF. E. B. POULTON has sent us the following account 

 •^ of a discovery of great interest to naturalists made by Mr. 

 A. H. Thayer, and a paper in which Mr. Thayer himself 

 describes his observations and conclusions. This paper has 

 been specially revised for publication in Nature, and with 

 Prof. Poulton's introduction will be welcomed by many ob- 

 servers of nature. 



No discovery in the wide field of animal coloration has been 

 received with greater interest than Mr. Abbott II. Thayer's 

 demonstration, by means of models presented to the Natural 

 History Museums of London, Oxford and Cambridge, of the 

 cryptic effect of the gradation of animal tints, from dark on the 

 back to white on the belly. In spite of the intense interest 

 aroused in students of animal life from the side of art as well as 

 from the side of zoological .science, the underlying principles have 

 been frequently misunderstood. 



Mr. Thayer has seen some of the accounts of his discovery 

 which have appeared in this country, and he feels that the 

 explanation offered has been inadequate and sometimes mis- 

 leading. He has therefore sent for publication in Nature a 

 further statement, which may be regarded as an appendix to 

 his original memoirs in The Aiii for April and October, 1896. 

 In this statement he makes a too generous acknowledgment of 

 my partial discovery of the same principle (unknown to him in 

 1896) in two isolated cases in 1SS6 and 1S87. I should wish, 

 therefore, to state that I did not discover, and could never have 

 discovered, what it required the eye of an artist to see — viz. the 

 manner in which the total colour-effect of the cold white under 

 side of an animal bathed in shadow and yellow earth reflections 

 matches exactly its earth-brown back bathed in the cold blue- 

 white of the sky. I furthermore failed to see the wide applica- 

 tion of that part of the principle which I did discover, and 

 not only failed to see it, but actually applied to the white under 

 sides of animals and the white eggs of certain birds the 

 erroneous interpretation which was then commonly received, 

 the interpretation which Mr. Thayer disposes of so completely 

 in the article printed below. 



The following account was drawn up by the present writer 

 for the models presented by Mr. Thayer to the Oxford Uni- 

 versity Museum. It is believed that the description of the 

 principles concerned may be useful to students in other museums. 

 I should add that Mr. Thayer cordially approves this descrip- 

 tion of the principles he has discovered : — 



" Models to show the manner in which wild animals are 

 commonly hidden. 



" Made and presented by Abbott H. Thayer, Esq., of Scar- 

 bro, N.Y., U.S.A. 



" If the two model ducks in this case be looked at from a 



NO. 1695, VOL. 65] 



little distance, the left-hand model will appear almost invisible, 

 transparent and ghost-like, while the right-hand one stands out 

 in startling contrast. The former has a colour arrangement 

 similar to that commonly found among wild animals in nature, 

 while the latter is entirely different. 



" There are two quite distinct elements in the concealment of 

 the left-hand model, and of such an animal as a hare or a wood- 

 cock. First there is loss of all appearance oi solidity, secondly 

 there is the harmony with the colour of the background. 



"We are led to believe that any small object is W«V/ and 

 possesses a definite shape solely because of the varying depth of 

 shade on parts of its surface more or less shielded from light. 

 (In the case of a very large object, such as a mountain, the 

 adjustment required when the eye focusses its near and distant 

 parts may also aid the judgment, but this could not apply to 

 anything so small as an animal), Thus an artist can make an 

 object on the flat surface of his canvas appear to stand out as a 

 solid because he paints the shadows as they would be caused 

 by the varying degree of light on the surface of a solid. Mr. 

 Thayer has shown for the first time that the opposite operation 

 is quite possible, viz. that an artist can paint a solid object so 

 as to obliterate the shadows and as a result to remove all appear- 

 ance of solidity. In the case of an object illuminated, as 

 animals are in nature, by the direct and reflected light of the 

 sky, this is achieved by colouring the object darkest on the top 

 where the light is strongest, gradually less dark on the sides 

 where the light progressively lessens, and white underneath 

 where the light is least, the darkening of the colour correspond- 

 ing exactly to the strengthening of the illumination. This will 

 be seen at once by turning the handle at the side of the case. 

 The right-hand model is, on the other hand, of uniform colour, 

 and appears far darker on the sides than the back, and darkei 

 still, almost dead black, underneath. 



" In fact the model which is the same shade of colour all 

 over appears to be a different shade everywhere because of the 

 difference in illumination ; while the model which is of a 

 different shade at every level appears to be the same shade all 

 over because the differences of shade exactly counterbalance the 

 differences of illumination. 



" Animals in nature are commonly graded in colour like the 

 left-hand model ; and Mr. Thayer's discovery of this great and 

 yet simple principle was made because he, as an artist, recognised 

 the ghost-like appearance of wild animals and then set to work 

 to analyse its cause. 



" But the obliteration of solidity would not effectually conceal 

 if the colour did not harmonise with the environment. The 

 back of the model, and of animals generally, is of the same tint 

 as the brown of the earth bathed in the cold blue-white light of 

 the .sky ; the under side of the model and the belly of animals is 

 of a cold blue-white bathed in shadow and yellow earth 

 reflections. These two mixtures produce colour effects which 

 are similar to each other and to the mixtures of intermediate 

 components on the sides. 



" Hence with solidity eliminated and with colour harmony 

 between environment and object, the latter appears to be but a 

 part of the former. It is thus possible to explain the conceal- 

 ment of the left-hand model, or of such an animal as the hare 

 crouching motionless on bare earth, or the numerous sand- 

 coloured quadrupeds, birds and reptiles of the desert ; but upon 

 the surface of most animals markings are added which suggest 

 the details of a more varied environment, such as that presented 

 by masses of brown leaves, twigs, reeds, grasses, lichen, i:c. 

 It is obvious th,it in an environment full of varied detail a 

 colouring producing a uniform effect would not conceal ; hence 

 the markings on the woodcock, ptarmigan, &c. In such cases 

 the animal itself appears to become part of the background 

 while its markings are seen as the details. 



" Mr. Thayer has also gained further proof of the accuracy 

 of his interpretation by painting out the gradation of colour on 

 the sides and belly of a stuffed bird, thus extending the colour 

 of the back over all parts ot the surface. Although a living 

 bird with its natural colouring would be almost invisible in 

 nature, the painted specimen became extremely conspicuous 

 when placed in the natural attitude and amid natural 

 surroundings. 



"It is not too much to assert that the bro.id fact of the 

 colour gradation on the sides of animals passing into white 

 underneath has now for the first time received its inter- 

 pretation. "Edward B. Poulton. 



"Oxford, January 22, 1902." 



