42 INFLUENCE OF LIGHT 



To add to these beauties there is seen in certain 

 lights a rich flush of iridescent purple reflected from 

 the whole surface of the animal. 



A few hours' captivity changes all this, and the 

 Prawn, though it does not appear to have suffered in 

 health or vigour, has put on a most quakerly sobriety 

 of colour, all the fine bands and stripes and spots 

 having become so pale as to be scarcely distinguish- 

 able from the general pellucid olive hue of the body. 



I cannot tell how this loss of colour is effected 5 

 but I have reason to think that light, the great agent 

 in 'producing colour in most case's, is the cause. I 

 took two specimens just dipped from a deep poo], and 

 equal in the richness of their contrasted colours ; one 

 of these I placed in a large glass vase of sea-water 

 that stood on my study-table ; the other in a similar 

 vase shut up in a dark closet. In twenty-four hours 

 the one that had been exposed to the light had taken 

 on the pale appearance just alluded to ; the one that 

 had been in darkness had scarcely lost any of the 

 richness of its bands and stripes, though the general 

 olive hue of the body had become darker, and of a 

 brow^ner tint. This individual, however, assumed the 

 appearance of the former, before it had been an hour 

 emancipated from its dark closet. Without attempt- 

 ing to account for the phenomenon, I would just 

 advert to the parallel exhibited by the sea-weeds. 

 The brilliant colours displayed by many of these 

 exist, as is well known, in the greatest perfection, 

 when the plants grow at considerable depths, or in 

 the caves and holes of the rocks, where light can but 

 very dimly penetrate. Some of these will not grow 



