232 Dr. H. Dor on Vision in the Arthropoda. 
to it, to the bacillar coat; and lastly, the optic ganglion to those 
layers of the retina which are composed of granulations, cells, 
and nerve-fibres. The pigments are the analogue of the choroid 
and the iris; and the transversely striated muscular fibres have 
their equivalents in the muscular elements of the choroid and ins.” 
The researches of Leydig were followed by investigations of 
the anatomy of the compound eye by Zenker, Gegenbaur, and 
Leuckart, and on its embryology by Claparéde. Zenker found 
the curvature of each facet of the eye in Dytiscus to equal 160°. 
The index of refraction of the cornea = 1°50, and that of the 
the crystalline body = 1:40. He concludes that the vitreous 
body is placed behind the cornea only to prevent the convergence 
of the rays behind the cornea, so as to form an image before 
reaching the summit of the crystalline body. In this view, 
opposed to Leydig, the compound eye is an aggregation of sim- 
ple eyes. Gegenbaur, on the contrary, adopts Leydig’s view, 
and thinks he has demonstrated an uninterrupted communica- 
tion of the crystalline body and nervous fibre with the optic 
ganglion in a Hyperid Crustacean. 
Leuckart, on the other hand, not content with the various 
hypotheses amongst which he had to choose, adopts another, 
still more difficult to understand. He states that between the 
cornea and the crystalline body there is a space containing a 
vitreous gelatinous body enclosed in a proper envelope. Some 
muscular fibres found in this envelope may, by their contraction, 
approximate the crystalline body to the cornea ; in other words, 
there exists a special organ of accommodation. Besides all this, 
in the genera Sapphirina, Coryceus, &c., the crystalline is said 
to be composed of two distinct parts, exactly like the crown and 
flint glass in achromatic lenses. These observations the author 
has been unable to verify. Leuckart compares each facet of 
such eyes, not to a camera obscura, but to a telescope with a 
simple object-glass (cornea) and eye-piece (posterior lens) ; and 
he considers this view to be the more probable as movements of 
the posterior lens may be observed which can have no other ob- 
ject than to adapt the apparatus to different distances. 
Claparéde thinks Miller’s theory untenable, and also cites 
some facts which speak against Leydig’s views; but he avoids 
giving his own. 
This being the state of the question, the author endeavoured, 
by fresh researches, to solve the problem, How is vision effected 
in the Arthropoda? For this purpose it was necessary to con- 
firm or refute Leydig’s theory, Miiller’s being already upset by 
the observations of Leuwenhoeck ; whilst Leuckart’s seemed un- 
tenable to him, because, if insects really possess a telescope, they 
would require an eye behind it to enable them to see. It is by 
optical processes that he attempts the solution of his problem. 
