VISION. 439 



simple tube, furnished with all the elements requi- 

 site for distinct vision, and capable of receiving im- 

 pressions from objects situated in the direction of 

 the axis of the tube. The rays traversing adjacent 

 corneules are prevented from mixing themselves 

 with those which are proper to each tube by the 

 interposition of the black pigment, which com- 

 pletely surrounds the transparent cylinders, and 

 intercepts all lateral or scattered light. Thus has 

 nature supplied the want of mobility in the eyes of 

 insects, by the vast multiplication of their number, 

 and by providing, as it were, a separate eye for 

 each separate point which was to be viewed ; and 

 thus has she realized the hypothetical arrangement, 

 which suggested itself in the outset of our inquiries, 

 while examining all the possible modes of effecting 

 this object. 



This mode of vision is probably assisted by the 

 converging powers of each corneule ; although in 

 parts which are so minute it is hardly possible to 

 form an accurate estimate of these powers by direct 

 experiment. In corroboration of this view I am 

 fortunately enabled to cite a valuable observation 

 of the late Dr. WoUaston, relative to the eye of the 

 Astaciisjiuviatilis, or cray-fish, where the length of 

 each component tube is short, compared with that 

 of the Libellula. On measuring accurately the 

 focal distance of one of the corneules. Dr. WoUas- 

 ton ascertained that it corresponds with great exact- 

 ness to the length of the tube attached to it ; so that 

 an image of an external object is formed precisely 

 at the point where the retina is placed to receive it.* 



* This interesting fact was communicated to me by the late Cap- 

 tain Kater, who, together with Mr. Children, assisted Dr. Wollaston 

 in this examination. 



