1913] on Reflection in Aquatic and Sub-Aquatic Life 783 



red and brown, but when seen airainst 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 l)reakwater 

 at Port Erin. 



Lobsters, crabs, and many other forms of marine life, usually 

 found in crevices among dark rocks covered with red and brown 

 seaweeds, show a pigmentation exactly similar in appearance to the 

 colour reflected from the seaweed. This is particularly well marked 

 in the swimming crab, Portimus puher. 



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 occupies on the surface, rela- 

 tively to the point of observation from below the water. 



On looking up to the surface, an observer sees above him a 

 circle of ligh^, 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 

 of light and beyond. 



Now it is generally understood that forms of life that occasionally 

 or habitually float on the surface are 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 trans- 

 lucent. 



Commencing with the opaque, I will illustrate the point with 

 the appearance 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 is not concealed against the clouds at any time, 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, 

 howevei', concealed by reflection in the area of total reflection. 



How does this explanation affect the concealment of a white 

 opaque 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 



