VISION. 491 



vast multiplication of their number, and by pro- 

 viding, as it were, a separate eye for each sepa- 

 rate point which was to be viewed ; and thus 

 has she realized the hypothetical arrangement, 

 which suggested itself in the outset of our in- 

 quiries, 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 pos- 

 sible 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 Astacus fluviatilisy or 

 cray-fish, where the length of each component 

 tube is short, compared with that of the Li- 

 bellula. On measuring accurately the focal 

 distance of one of the corneules, Dr. Wollaston 

 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.* 



Little is known of the respective functions of 

 these two kinds of eyes, the simple and the com 



* This interesting fact was communicated to me by Captain 

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

 in this examination. 



