128 Discoveries ofMilller and others in the 



of the second species, the distinctness of the image will be the 

 greater, precisely in proportion as all rays not perpendicular 

 are the more perfectly excluded. For this purpose an organ 

 will be required, which, situated in front of the spherical 

 retina, shall accurately effect this separation of the perpendi- 

 cular from the non-perpendicular rays. 



The visual organ of the second kind is so simple, and so 

 completely in accordance with the known laws of the motion 

 of light, that we might readily anticipate its existence. When 

 we know what is essential in such a structure, it is very- 

 easy. to understand every modification of which it is suscept- 

 ible. The essential conditions are : that the retina be sphe- 

 rical; that the light traverse a transparent medium in the 

 direction of the rays of such sphere ; and that this medium 

 be so disposed as to intercept all light not approaching in 

 exact parallelism with these rays. The compound eyes of 

 insects contain indeed all the organs necessary for an eye not 

 seeing by refraction ; and these organs have been hitherto 

 almost entirely neglected in anatomical researches upon this 

 subject. Its organisation accords perfectly with the general 

 laws now stated. 



A visual organ of the second species, when it is double, 

 must be immovable ; and these double or multiple eyes ought 

 to have divergent axes. For, if the retinae are to be impressed 

 only by the rays coming perpendicularly to their surface, it 

 will be necessary that two spherical eyes form segments of 

 only one sphere ; and the axes of the two eyes ought never 

 to converge, for, wijthout this condition, the luminous parts 

 of objects situated at the point of convergence would neces- 

 sarily be visible to both eyes, and double vision would be the 

 result. 



In the anatomical details now to be given, the simple eyes 

 or stemmata will be first described, and the description will 

 be accompanied bya slight account of the vision possessed 

 by animals provided with eyes of this kind. The compound 

 eyes will then be treated in a similar manner. 



TJie Simple Byes of Spiders, Scorpions, Sfc, and the Stemmata, 



of Insects, 

 ilfygale «vicularia. — The transparent cornea of this spider 

 is circular, convex, and of firm consistence : it is a direct con- 

 tinuation of the common integuments, but of course altered 

 in texture. Immediately behind the cornea is found a yel- 

 lowish, hard, and nearly spherical crystalline lens : its 

 convexity appears to be the same anteriorly and posteriorly ; • 

 and its posterior surface is closely applied to a spherical body, 



