A fly's eye. 161 



hollow cones of the blow-fly are filled with ca watery or albuminous 

 fluid. This would not refract light very much; so, possibly, they 

 are hardly lenses. 



Thus far, then, I think our understanding of a fly's eye is 

 clear — we find a series of lenses (the cornea)^ each forming an 

 optical image, and a corresponding set of modified crystalline 

 lenses beneath them. The focus of this doublet lens falls just 

 at the point of the " cone " (see Fig. 5). So here we ought to 

 look for the sensitive plate, or portion of the eye which receives 

 the picture. It is not a plate, but a little knob, the end of a 

 nerve fibre, and close round it we find cell structure which 

 comparative anatomy tells us must be of a highly sensitive nature. 

 In a very little space there is quite a crowd of nuclei^ and the 

 nucleus is the living, the most active part of a cell. Each cone is 

 surrounded by opaque pigment, generally burnt-sienna-coloured, in 

 flies, and at the points of the cones are rings of brighter pigment, 

 the colour of which is sometimes brilliant crimson (/^./., Fig. 4, and 

 pg.c.^ Fig. 5). Now there are nuclei in the pigment surrounding 

 the cone, nuclei in the rings of pigment at the point of the cone, 

 nuclei round the little knob, and nuclei again round the " rod '' of 

 which the knob is the termination; n"-^ ;r, 71^, ?i^, Fig. 5. Such 

 a part must therefore be extremely sensitive, and the presence of 

 the pigment is said to show that the structure is sensitive to light ; 

 for, I am told, without the opaque pigment light would pass 

 through the transparent nerve fibre without aftecting it. At these 

 points, therefore, are formed optical images on sensitive tissue, one 

 image by every lens. Our own two eyes give us two images ; the 

 two eyes of a fly give it some two or three thousand images. By 

 the power we possess of moving our eyes we are able to make 

 their two images overlap, so that we only perceive one. For the 

 eyes of a fly to be useful, it is clear that it must possess some 

 means of combining the multitude of images into one perception, 

 and this means is supplied by the peculiar structure of the retina. 



Immediately beneath the cones in a fly's eye stands a series of 

 rods, fitting to them by the knobs before mentioned. (I may 

 remark that it is very unfortunate for these structures to have 

 been called " rods and cones," because the name tends to con- 

 found them with the " rods and cones " of a mammalian retina. 



