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. Wollaston, 

 relative to the eye of the Astacus fluviatilis, 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. 



