VARIOUS OPINIONS. 55 



Sir David thought that the experiments required to be 

 repeated by persons acquainted Avith those branches of 

 physical optics with Avhich the phenomena were inti- 

 mately allied. It is very easy to explain why a fish 

 may appear dark in a dark vessel, and light in a co- 

 loured 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 pecuHar colours of fishes depend on 

 the tliickness or size of certain minute transparent par- 

 ticles, 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 mechanical 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 physiolo- 

 gical 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 fihus, which are quite 

 different when they are dry from what they are when 

 immersed in a fluid. 



*'A conversational discussion then took place, in which 

 Professor Connell supported Dr. Gillespie's views, and 

 Dr. Reid those of Sir D. Brewster." 



This subject is in such good hands, that I shall not 

 intrude any speculative observations of my own. We 

 have lately seen such wonderful effects produced by 



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