784 Dr. Francis Ward [June 6, 



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 sea-gulls on the surface within 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 ray 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 hke the white saucer. 



In the area of total reflection, however, the nautilus appears 

 white, for in consequence of not being an opaque object it is no 

 longer a reflector. 



The nautilus seems to slip between two stools, it is too opaque to 

 be concealed in the circle of light, and too translucent to be con- 

 cealed in the area of total reflection. In the latter situation it 

 certainly may be protected by simulating the appearance of wave 

 foam, for wave foam in the area of total reflection appears as a 

 flicliering 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 edge of the circle of light, and the parts respectively above and 

 below appear to be separated by a considerable interval of water surface. 



When the portion above the water is white, as in a gull, it is 

 difficult to detect against the sky. 



The above remarks only refer to open water, and I will illustrate 

 how a wading bird is concealed against the image of a reed-bed 

 many yards behind him. 



Looked at from below the markings on a heron are in bold 

 upright lines, for the plumage is greyish white with black patches 

 on each side of the head, and the black patches on the shoulders 

 appear continuous with the black primaries on his wings. Seen 

 against an open sky, the white parts of the wading heron blend with 

 the sky, but the black parts stand up in bold relief. The head and 

 shoulders of the bird are seen on the edge of the circle of light, but 

 so also is the reed-bed many yards behind. The tops of the reeds 

 appear as perpendicular images, and the perpendicular markings on 

 the heron blending with them, make the bird inconspicuous. 



I have referred to white as a concealing agency. Black objects, 

 when they retain air-bubbles on their surface, also become reflectors 

 under the water. The black water-spider under a leaf appears 

 green and is lost to sight. A water-hen swimming on the surface 

 in the area of total reflection reflects the green weeds below, and 

 becomes difficult to discern against the surface which is reflecting 

 the same colour. 



