File:Hardwicke's science-gossip - an illustrated medium of interchange and gossip for students and lovers of nature (1884) (14783273555).jpg

Identifier: hardwickesscienc20cook (find matches)
Title: Hardwicke's science-gossip : an illustrated medium of interchange and gossip for students and lovers of nature
Year: 1884 (1880s)
Authors: Cooke, M. C. (Mordecai Cubitt), b. 1825 Taylor, J. E. (John Ellor), 1837-1895
Subjects: Science Natural history
Publisher: London : Robert Hardwicke
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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om decisive. No image is formedwhere sensory elements are present to receive andtransmit it. Moreover, the image is that of an ob-ject very near to the cornea, whereas all observations seven, and often fall as low as four —further, that therods in each group are often more or less completelyfused so as to resemble simple structures, and thatthis is especially the case with insects of keen sight.*Certain facts described by Schultze tell on theother side. Coming to the Arthropod eye, fresh fromhis investigation of the vertebrate retina, Schultzefound in the retinal rods of insects the same lamellarstructure which he had discovered in Vertebrata. Hefound also that in certain moths, beetles andCrustacea a liundle of extremely fine fibrils formedthe outer extremity of each retinal or nerve-rod.This led him to reject the mosaic theory of vision,and to conclude that a partial image was formedbehind every crystalline cone, and projected upon amultitude of fine nerve-endings. Such a retinula
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Fig. 155.—Section through simple eye of Vespa. The referencesas above. (Simplified from Grenacher.) Fig. 156.—Diagrammatic section of compound eye. Thereferences as above. of living insects show that the compound eye is usedfor far sight, and the simple eye for near sight.Lastly, the treatment with acid, though unavoidable,may conceivably affect the result. It is not certainthat the cones really assist in the production of theimage, which may be due to the corneal facets alone,though modified by the decolorised cones. Grenacher has pointed out, that the composition ofthe nerve-rod furnishes a test of the mosaic theory.According as the percipient rod is simple or complex,we may infer that its physiological action will besimple or complex too. The adequate perception ofa continuous picture, though of small extent, willrequire many retinal rods ; on the other hand, asingle rod will suffice for the discrimination of abright point. What then are the facts of structure ?Grenacher has as

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