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lighter; when the light is diminished the chromatophores expand and the colours 

 become darker. It is evident that with the same amount of illumination the alteration 

 of the colour and composition of the ground on which the sole rests alters the quantity 

 of light acting un the fish. For if the ground is black all the light whicli falls upon 

 that ground is absorbed, and the sole is only affected by the rays which fall directly 

 upon it, while if the ground is light-coloured nearly all the light which falls upon it is 

 reflected, and the diffused light which falls upon the sole is therefore greatly increased. 

 Just as a room is n\uch lighter with the same windows if its walls are white than if 

 they are black. 



The sole does not become uniformly coloured on a uniformly coloured ground. In 

 some of the laboratory tanks we have at the bottom a dark grey fine sand brought 

 from a part of the sea shore. This sand is extremely uniform in colour, and is exactly 

 of the same kind as the sand brought up by the trawl from the trawling grounds ofl' 

 Plymouth and off Mount's Baj^. In the tanks the soles usually have some of this sand 

 on their skins, and are therefore by no means conspicuous, but whenever their skins 

 are visible it is seen that the black markings are pronounced. The ground colour of 

 the soles in this condition approximates closely to that of the sand, but the presence of 

 the black spots does not seem to me to aid in the resemblance at all. I placed a large 

 sole in a shallow tub on some of this sand in order to make a careful observation. 

 After three davs the colouring of this sole was as follows : The intermediate black spots 

 were almost invisible ; of the dorsal series the second was faint, the third, fourth, and 

 fifth well marked and black ; of the median series only the third and fourth were 

 well marked, but not black. The ventral spots were all visible but faint ; termination 

 of the pectoral reddish-brown, white spots alternating with black quite distinct. 



