428 NATURE AND MAN. 



organ of vision, by what we encounter as we proceed upwards. 

 The next stage consists in the addition of something like a crystal- 

 line lens — a little, bright, pellucid particle on the end of the nerve- 

 fibre, that seems by the concentration of luminous rays to intensify 

 the sensation of light We have strong reason to believe that 

 animals very low in the scale are guided by this sensation ; not in 

 the manner of plants, whose growth towards light is accounted for 

 by its physiological action on the formation of their tissues ; but 

 in f7iovements directed by a conscious perception of light, resembling 

 that of a nearly blind person who can just distinguish light from 

 darkness. We find this direction towards light, and the avoidance 

 of intervening obstacles, more and more obviously manifested in 

 the movements of animals, as we pass upwards to higher forms of 

 the visual organ. In front of the crystalline lens, we meet with a 

 transparent film representing a cornea, separated from it by an 

 anterior chamber ; and behind it we come to distinguish a vitreous 

 humour, covering an expansion of the nerve-fibre which is backed 

 by a pigment layer. When we have arrived at this stage, seen in 

 the " simple eyes " of insects, it is most beautiful to trace how the 

 further ascent takes place along two distinct lines; one culminating 

 in the " compound eye " of the insect, and the other in the single 

 eye of the vertebrate animal, of which that of the predaceous birds 

 is, perhaps, the highest type. 



The " compound eye " of the insect, as you all know, is, in 

 its typical form, an almost hemispherical mass projecting from 

 the side of the head, which is made up of a number of separate 

 " eyelets " of nearly cylindrical form, whose several axes are 

 directed radially towards the spheroidal surface. Each " eyelet " 

 consists of a number of different components which appear to 

 correspond with those of our single eye ; probably giving an 

 achromatic character to the minute picture formed by its refrac- 

 tive action. But each can receive only those rays of light, 

 whose direction corresponds with that of its own axis ; and as 

 the eye of the insect is immovable, no eyelet can be made to turn 

 towards any particular object. By the multiplication of these 

 eyelets, however, and the radial direction in which they are fixed, 

 the aggregate "compound eye" will have a range fully equal, 

 and probably superior, to that of any single eye constructed on 



