COLOUR VARIATION 57 



as resulting from the same principle of preservation 

 by which the ptarmigan and alpine hares have 

 their colours changed with the approach of snow. 



"Notwithstanding these distinct statements by 

 so many observers in whom confidence might be 

 placed, Sir David thought that the experiments 

 required to be repeated by persons acquainted with 

 those branches of physical optics with which the 

 phenomena were intimately allied. It is very easy 

 to explain why a fish may appear dark in a dark 

 vessel, and light in a coloured one ; and why it 

 should have a still different appearance when taken 

 out of both vessels and exposed to the light of 

 the sun. All bodies assume the colour of the light 

 which they reflect, and a brilliant light will develope 

 colours which are invisible in light of ordinary 

 intensity. As the peculiar colours of fishes depend 

 on the thickness or size of certain minute trans- 

 parent particles, it is not easy to understand how 

 the fish could voluntarily alter the size or thickness 

 of those particles, or how exposure to another 

 colour could permanently produce the same me- 

 chanical effect. If a fish is kept in mossy or muddy 

 water, it will doubtless absorb the colouring matter 

 which the water may contain ; but this is rather a 

 process of dyeing than one of physiological action. 

 The changes said to take place in the colour of 

 fishes when dying might arise from the drying of 

 their scales, which produces a change in all colours, 

 but particularly in those of thin films, which are 

 quite different when they are dry from what they 

 are when immersed in a fluid. 



" A conversational discussion then took place, in 



