with Special Reference to the Vision of Insects. 329 



total reflexions. Now, it seems probable that something of 

 this kind actually occurs in the insect's eye. In fact the 

 apparatus corresponding to the rods and cones of our eyes 

 seems, so far as I can make out, to be situated, not at the 

 primary surface where the image is first formed, nor even at 

 the inner surface where the image may be reproduced in the 

 way described above, but in a deeper situation with which 

 the inner surface communicates only through curved trans- 

 parent threadlets. Each of these threadlets seems to have a 

 thin transparent core, and if this core be of sufficiently highly 

 refractive material, it would, although curved, be competert 

 to carry the light forward by total internal reflexions, from 

 the lower end of one of the glass threads to one of the pieces 

 of the apparatus which corresponds to the layer of rods and 

 cones in our eyes*. 



It is, perhaps, worth observing that an eminently useful 

 adiustment which we cannot effect seems to be possible in an 

 insect's eye. In fact, the inner surface of our model might 

 be drawn inwards more at one place than another ; and I am 

 disposed to think that muscles, acting on the inner surface, 

 are in the insect so disposed as to make this possible in its 

 eye. Now this would effect an accommodation to the dis- 

 tances of objects which would differ in the different parts of 

 uhe field of viewf. Moreover, this result may be brought 

 about in another way. The lenses and the funnels posterior 

 to them vary in size from one part of the compound eye of a 

 dragonfly to another, being largest in the position which I 

 suppose to be about the middle of the eye, and gradually 

 dwindling to about half this size rear the margin J. An 

 equable contraction of the " inner surface" of such an eye 

 would obviously effect a different accommodation in different 



* The light is probably carried forward most effectually where, as 

 in the dragonfly, the cores are less than a micron in section, i. e. not 

 much more than the wave-lengths of the light that has to traverse them. 

 Light would adapt itself to the sinuosities of such filaments, like sound 

 in a speaking-tube. 



t In the sections of the eyes of dragonflies, which I have examined, 

 the filaments from the funnels down to the " inner surface " are enclosed 

 within a sheath of fibres and are straight, but immediately after passing 

 through the inner surface they are each apparently enclosed within a 

 tube, and grouped in bundles, between which are open spaces which 

 may, perhaps, in the living insect have been occupied by muscles. 

 Muscles, in this situation, would be competent to effect the optical ad- 

 justment spoken of in the text. (See fig. 3.) 



% The increased aperture of the lenses towards the middle of a dragon- 

 fly's eye, and the diminished curvature of the stratum in which they lie, 

 both conduce to make its vision more perfect towards the middle of its 

 field of view ; and as this lies in the direction of the insect's flight, the 

 arrangement must be of advantage to it in its pursuit of prey. 



