THE COMPOUND EYES OF INSECTS. 267 



image is afforded by the insect's compound eye, contrary to the 

 generally expressed opinion. Then, again, the difficulty— hitherto 

 unexplained — why the facets differ so much in size, even in the 

 same specimen, seems possible of solution from the following 

 circumstance. In using pin-holes to. form images, it is well 

 known that, although all objects are in focus, still to obtain the 

 best possible results a certain relation must exist between the 

 diameter of the hole, the distance of the screen, and perhaps 

 the relative nearness of the object ; and this relation should 

 be strictly observed if the very finest definition be required. I 

 suggest, then, that the reason for the variation of the diameters of 

 the facets is to enable the insect to possess a differential selection 

 of optical arrangements ; so that if one set of holes did not 

 furnish sharp results, one of the many movements of its body — 

 intentional or otherwise — would facilitate the employment of a 

 different group of holes, and the desired result would be brought 

 about. 



Again, if this pin-hole theory be accepted by the entomologist, 

 it is needless to point out that the difficulty presented to the 

 physicist — namely, the entire absence of all appearance of strain- 

 effect in the cornea — disappears, for such would not be likely 

 to appear if the little holes were filled with a non-refractive 

 material. There is, however, one other point not yet touched 

 upon. The facets certainly generally appear in the specimens 

 of cornea as seen by the microscopist to possess a strongly - 

 convex front. This is so ; but in modern works upon the 

 subject it is distinctly stated L hat in a great number of instances 

 this is not really the case. It should also be borne in mind 

 that the appearance may be merely a post-mortem effect. W e 

 have only to recollect the flattening which the specimen under- 

 goes in order to make it lie beneath the cover-glass, to understand 

 that this might be a contributory cause. Then it is only 

 necessary to suppose the substance constituting the walls of 

 the holes to be of a more contractile nature than the material 

 which fills them, and the same effect would be produced. 



