1913] on Reflection in Aquatic and Sub-Aquatic Life 781 



have established at Port Erin a large wooden tank above ground. 

 Three sides of the tank incline at an angle of 45° ; the fourth is 

 perpendicular. In the perpendicular side is a glass window, and 

 attached to it on the outer side is an observation box, 6 feet by 4. 

 The tank and observation box revolve together on a platform ; by 

 this means an object in the tank can be seen by reflected and trans- 

 mitted light at will. 



It is usual to consider pigmentation as the important factor in 

 the concealment of sub-aquatic life. Among organisms that live in 

 more or less the same character of surroundings, this is true to a 

 great extent ; but in the forms of life that are constantly changing 

 their environments, the best concealed are those that most effectively 

 reflect their surroundings. 



When, however, an organism depends mainly upon reflection for 

 its concealment, the reflection of light from above has to be modified, 

 or else the organism is revealed. In some forms of Hfe, particularly 

 fishes, pigmentation upon the back is the method of modifying this 

 reflection from above. In other forms this top light is cut off by 

 position, e.g. in light-coloured anemones, which are only to be found 

 attached to the under-surface of shelving rocks. 



Before proceeding further, I would like to illustrate the appear- 

 ance of a white object, as seen from under the water. 



I show a sheet of white cardboard pinned on a red stick, which in 

 turn was stuck in the centre of an empty pond. The sides and 

 bottom of the pond were covered with green conferva. In this 

 position the card appeared white, and incidentally the stick red. 



The pond was then filled up with water, and now the white card 

 so exactly reflected the colour around, that it became practically 

 invisible, yet its position was revealed by a streak of light along the 

 upper edge of the card. 



In nature all white sub-aquatic organisms reflect in a similar 

 manner, and white is never seen under the water, except when there 

 is no provision made for modifying the reflection of light from above, 

 or when the organism turns on its side. 



As an illustration of this point let us consider the white anemone 

 {Actinoloha diaMhus). I show a colour-plate of this anemone attached 

 to the top of a rock and, of course, it appears white, but as soon as 

 it moved only a distance of two inches under the shelving edge of the 

 rock, the top light was cut off, and you will see the white anemone 

 appears green as it reflected the prevailing colour below. 



It will be noticed that the white serpula on the rock reflects in 

 the same manner ; and as the light parts of rock also appear of a 

 greenish colour the anemone and the serpula are practically invisible. 



I say practically invisible advisedly, for the greenish anemone 

 when closed makes a uniformly shaded green mass against a patterned 

 rock. 



It will have been noticed that the red stick holding the white 



