598 



NATURE 



[August 7, 19 r 



•counter-shaded bird, or beast, without markings, 

 -when seen against a patterned background, becomes 

 conspicuous, because it interrupts the pattern. The 

 same is seen in the fish world, and in illustration I 

 would direct your attention to the appearance of a 

 perch (I'crca fluviatilis) swimming past a reed bed. 



In the autochrome of a brown trout lying under 

 a stepping-stone, I show the value of reflection ; 

 here, the back green, and the belly red, as they 

 reflect the stones above and below, are undoubtedly 

 the main factors in concealing this fish, and the mark- 

 ings simply prevent the body from appearing pattern- 

 Jess. 



I would next direct your attention to the possible 

 influence of reflection of light, from some forms of 

 marine vegetation, upon the pigmentation of various 

 marine organisms. Several red and brown seaweeds 

 seen by transmitted light appear red and brown, but 

 when seen against a dark background they reflect 

 at various points a brilliant bluish-purple colour. 

 Chondrus shows this well. In certain positions the 

 whole side of a dark rock, covered with red and 

 brown seaweed, shows blotches and streaks of bluish- 

 purple. This is well marked upon the concrete blocks 

 •on the old breakwater at Port Erin. 



ance of a thick white saucer. This was floated from 

 some distance over my head. Outside the circle of 

 light the surface of the water was reflecting the green 

 bottom of the pond, the white saucer did the same, 

 and, therefore, was invisible against the surface of the 

 water. When, however, it came into the circle of 

 light it still reflected the dark colour below and was 

 revealed as a well-defined dark object against the 

 sky and clouds. 



A white-breasted gull swimming on the surface is 

 concealed and revealed in an exactly similar manner. 

 Therefore, an opaque white organism in the circle of 

 light is not concealed, and when seen against the 

 clouds the whiter the object the more conspicuous 

 it becomes, because it reflects the dark water below. 

 A white object is, however, concealed by reflection 

 in the area of total reflection. 



How does this explanation affect the concealment 

 of an opaque white object on the sea from a fish'' 

 The size of the circle of light on the surface depends 

 entirely upon how far the fish is under the water for 

 lines drawn from the two ends of the diameter of the 

 circle make an angle of 97 at the eye of the fish. 



When the fish is some depth under the water there 

 may be several white seagulls on the surface within 



Fig. 3. — Cormorant < 



Lobsters, crabs, and many other forms of marine 

 life, usually found in crevices among dark rocks 

 covered with red and brown seaweeds, show a pig- 

 mentation exactly similar in appearance to the colour 

 reflected from the seaweed. This is particularly well 

 marked in the swimming crab, Portunus fiber. 



I would now refer to the appearance of life on the 

 surface, as seen from below. This appearance entirely 

 depends upon the position that the particular organism 

 oci upies on the surface, relatively to the point of 

 •observation from below the water. 



On looking up to the surface, an observer sees 

 above him a circle of light, through which he can see 

 the sky and clouds. Beyond this circle there is total 

 reflection, and the surface of the water reflects the 

 general colour below. Transparent organisms are 

 practically invisible, both in the circle ol light and 

 "beyond. Now it is generally understood that forms 

 of life that occasionally or habitually float on the sur- 

 face arc white underneath, so as to conceal them 

 against the clouds and wave foam. 



In dealing with this subject it is necessary to make 



a difference between white organisms that are opaque 



■and those that are translucent. Commencing with 



the opaque, I will illustrate the point with the appear- 



NO. 2284, VOL. 9 1 J 



the circle of light, but as the fish comes up to feed 

 his circle of light is narrowed down, and the gulls 

 slip into the area of total reflection and by reflection 

 become invisible to the fish. 



For my experiments with translucent organisms I 

 used the shell of an argonauta. In the circle of light 

 you will see the shell is still very obvious, but as it 

 transmitted a considerable amount of light it did not 

 appear black like the white saucer. In the area of 

 total reflection, however, the shell appears white, 

 for in consequence of not being an opaque object it is 

 no longer a reflector. 



Argonauta seems to slip between two stools; it 

 is too opaque to be concealed in the circle of light, 

 and too translucent to be concealed in the area of total 

 reflection. In the latter situation it certainly may be 

 protected by simulating the appearance of wave foam, 

 tor wave foam in the area of total reflection appears 

 as a flickering light. 



So far we have only considered that portion of an 

 object that is actually immersed. If, however, the 

 organism under consideration is not too far distant, 

 that portion of it above the water is visible on the 



