164 A fly's eye. 



the lens of the eyelet. And these little pictures fit together in the 

 fly's perception like the little cubes of a mosaic picture, forming a 

 continuous whole. 



An illustration, which I think is true as far as it goes, may 

 help to explain to some extent how vision takes place. Supposing 

 nervous action to be something like electric action, the pictures 

 from the lenses, falling on the knobs of the rods, are like the 

 message rapped into his machine by the telegraphic operator ; the 

 nerve-fibres are like the telegraph wires ; and the nerve-junctions 

 are the galvanic batteries which alone render the transmission of a 

 current possible, and perceptible by the receiver at the other 

 end : — the " other end " in the fly being its brain. Here we are 

 brought up to the problem how nervous action becomes " states 

 of consciousness," and as that problem is insolvable, here we 

 must stop. 



To sum up : a fly's eye consists of many eyelets ; each eyelet 

 has a lens to form a picture, and a sensitive knob on which the 

 picture falls ; the many pictures of the compound retina are 

 transmitted by fibres through two nerve-junctions to the brain ; 

 and, in the course of their passage, they are combined into one 

 perception. One or two observations of a fly's habits will bring 

 this paper to a fitting close. Flies are decidedly short-sighted — 

 for they do not perceive you until you get within a few yards of 

 them. They probably have some perception of colour as well as 

 of form, for, when seeking honey for food, they alight with 

 certainty on a flower rather than on a leaf. Their sense of smell, 

 however, which is certainly acute, doubtless helps their sight in 

 this point. Yet their eyes as a whole show us that the faculty of 

 sight, always wonderful, is very wonderful indeed in our too 

 familiar friend, the Blow-fly. 



EXPLANATION OF PLATES XIV. AND XV. 



Fig. 4. — Section of a fly's eye :—!., lenses; co., cones; pt., pigment layers, con- 

 sisting of rings round the rods ; r.r., rods; a.v. 1, air-vessels between 

 the rods ; m. 1, membrane on which the rods and air-vessels stand ; 

 a.v. 2, short lengths of air-vessels, which form a matted layer above the 

 first nerve- junction ; n.j. 1, first nerve-junction ; m. 2, membrane on 

 which it stands; A.V., A.V., large air-vessels around the eye; 

 n.j. 2, second nerve-junction; a.v. 3, air-vessels; op.n., optic nerve; 

 t)-., brain; x 160 diameters. 

 ,, 5. — Single eyelet of a fly. The lettering is the same as in Fig. 4, with the 

 addition of n. 1, 2, 3, 4, nuclei; py.c, pigment-cells; nv., norve-fibros ; 

 and nv.c, nerve-cells ; x 600 diameters. 



