HAEDWICKE'S SCIENCE-G OSSIF. 



251 



alone. He finds, however, that the expeiifnent may 

 be successfully performed with eyes not liable to this 

 objection, e.g., the eyes of nocturnal lepidoptera. A 

 bit of a moth's eye is cut out, treated with nitric acid 

 to remove the pigment, and placed on a glass slip 

 in the field of the microscope. The crystalline cones, 

 still attached to the cornea, are turned towards the 

 observer, and one is selected whose axis coincides 

 with that of the microscope. No image is visible 

 when the tip of the cone is in focus, but as the cornea 

 approaches the focus, a bristle, moved about between 

 the mirror and the stage, becomes visible. This ex- 

 periment is far from decisive. No image is formed 

 where sensory elements are present to receive and 

 transmit 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 the 

 rods in each group are often more or less completely 

 fused so as to resemble simple structures, and that 

 this is especially the case with insects of keen sight.* 

 Certain facts described by Schultze tell on the 

 other side. Coming to the Arthropod eye, fresh from 

 his investigation of the vertebrate retina, Schultze 

 found in the retinal rods of insects the same lamellar 

 structure which he had discovered in Vertebrata. He 

 found also that in certain moths, beetles and 

 Crustacea a liundle of extremely fine fibrils formed 

 the 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 formed 

 behind every crystalline cone, and projected upon a 

 multitude of fine nerve-endings. Such a retinula 



Fig. 155. — Section through simple eye of Vespa. The references 

 as above. [Simplified from Grenacher.] 



Fig. 156.— Diagrammatic section of compound eye. The 

 references as above. 



of living insects show that the compound eye is used 

 for far sight, and the simple eye for near sight. 

 Lastly, the treatment with acid, though unavoidable, 

 may conceivably affect the result. It is not certain 

 that the cones really assist in the production of the 

 image, which may be due to the corneal facets alone, 

 though modified by the decolorised cones. 



Grenacher has pointed out, that the composition of 

 the 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 be 

 simple or complex too. The adequate perception of 

 a continuous picture, though of small extent, will 

 require many retinal rods ; on the other hand, a 

 single rod will suffice for the discrimination of a 

 bright point. What then are the facts of structure ? 

 Grenacher has ascertained that the retinal rods in 

 each element of the compound eye rarely exceed 



of delicate fibrils has received no physiological 

 explanation, but it is now known to be of com- 

 paratively rare occurrence ; it has no pigment to 

 localise the stimulus of light j and there is no reason 

 to suppose that an image can be formed within its 

 limits. 



The optical possibility of such an eye as that inter- 

 preted to us by Miiller has been conceded by phy- 

 sicists and physiologists so eminent as Helmholz and 

 Du Bois Reymond. Nevertheless, the competence of 

 any sort of mosaic vision to explain the precise and 

 accurate perception of insects comes again and again 

 into question whenever we watch the movements of a 

 housefly as it avoids the hand, of a bee flying from 

 flower to flower, or of a dragon-fly in pursuit of its 



* Flies, whose eyes are in several respects exceptional, have 

 almost completely separated rods, notwithstanding their quick 

 sight. 



