270 ANIMAL SKETCHES. CHAP. 



other rods are absorbed by the pigmented material. 

 Similarly with rays starting from any other point of the 

 arrow. Only those which, in each case, pass straight 

 down one of the rods reach the screen. Thus there is 

 produced a small stippled image c' d', of the arrow. 

 Lowne has experimented with fine glass threads, arranged 

 like the cones and nerve-rods of the bee's eye, and finds 

 that (even when they are not surrounded by pigment, as 

 are the elements in an insect's eye) all oblique rays are 

 got rid of by numerous reflections and the interference 

 due to the different lengths of the rays. Some modifica- 

 tion of the mosaic hypothesis is now generally adopted, 1 

 and Dr. Hickson has recently worked out, with great care, 

 the structure of the optic tract which lies between the 

 crystalline cones and the brain. 



Imperfect as our knowledge of the sensations of bees 

 may be and in a subject of such abstruseness and diffi- 

 culty we must expect imperfection we yet have no 

 reason to suppose that this is due to any imperfection 

 in their sensory endowments. There are three simple 

 eyes, useful, it is supposed, for near vision in the hive, and 

 a pair of large compound eyes for the ascertainment of 

 more distant space relations. These faceted eyes are 

 covered with delicate hairs which protect the facets from 

 extraneous particles, and from which such particles may 

 be removed by combs specially developed for that purpose 

 on one of the joints of the fore-leg. There are organs of 



1 The just-published observations of Prof. Exner have finally established 

 the truth of the mosaic hypothesis. Mounting the eye of a fire-fly in such 

 a way that the outer surface was exposed to the air, and the inner parts 

 with the cones were immersed in a fluid of the same density as the blood 

 of the insect, he has obtained, in a camera attached to the microscope, a 

 photograph of the window of the room to which the eye was turned and 

 of a church spire seen through the window ; not, that is to say, a multi- 

 plicity of images, but a single image for the compound eye as a whole. 



