MODE OF VISION BY FACETTED EYES 



257 



How vision is effected by a many-facetted eye is thus explained 

 by Lubbock : " Let a number of transparent tubes, or cones with 

 opaque walls, be ranged side by side in front of the retina, and 

 separated from one another by black pigment. In this case the only 

 light which can reach the optic nerve will be that which falls on any 

 given tube in the direction of 

 its axis." For instance, in Fig. 

 267,' the light from a will pass 

 to a', that from b to b 1 , that from 

 c to c', and so on. The light from 

 c, which falls on the other tubes, 

 will not reach the nerve, but will 

 impinge on the sides and be ab- 

 sorbed by the pigment. Thus, 

 though the light from c will 

 illuminate the whole surface of the eye, it will only affect the nerve 

 at c'. 



According to this view those rays of light only which pass directly 

 through the crystalline cones, or are reflected from their sides, can 

 reach the corresponding nerve-fibres. The others fall on, and are 

 absorbed by, the pigment which separates the different facets. 

 Hence each cone receives light only from a very small portion of the 

 field of vision, and the rays so received are collected into one spot of 

 light. 



It follows from this theory that the larger and more convex the 



FIG. 267. From Lubbock. 



eye, 



the wider will be its field of vision, while the smaller and 



more numerous are the facets, the more distinct will be the vision 

 (Lubbock). 



The theory is certainly supported by the shape and size and the 

 immense number of facets of the eye of the dragon-fly, which all 

 concede to see better, and at a longer range, than probably any other 

 insect. 



M tiller's mosaic theory was generally received, until doubted and criticised by 

 Gottsche (1852), Dor (1861"), Plateau, and others. As Lubbock in his excellent 

 summary states, Gottsche's observation (previously made by Leeuwenhoek) 

 that each separate cornea gives a separate and distinct image, was made on the 

 eye of the blow-fly, which does not possess a true crystalline cone. Plateau's 

 objection loses its force, since he seems to have had in his mind, as Lubbock 

 states, Gottsche's, rather than Miiller's, theory. 



Miiller's theory is supported by Boll, Grenadier, Lubbock, Watase, and 

 especially by Exner, who has given much attention to the subject of the vision 

 of insects, and is the weightiest authority on the subject. 



Gottsche's view that each of the facetted eyes makes a distinct image which 

 partially overlaps and is combined with all the images made by the other facets, 



