1878.] 



the Simple and Compound Eyes of Insects. 



267 



The semi- compound eye introduces no new difficulty in this theory, 

 it is probable, I think, that more than a single luminous impression is 

 received by the elements which are situated behind each facet, and 

 that these correspond with portions of the field of vision which are 

 remote from each other, the central rod cell of one facet correspond- 

 ing to one of the peripheral rod cells of some other facet; the 

 extreme complexity of the connexions between the cells of the 

 ganglionic retina renders this view not improbable. 



In order to determine the effect of the long fine highly refractive 

 threads of the eyes of insects upon the light, I made some experi- 

 ments on the transmission of light through fine threads of glass. 



I took a capillary tube of glass of an inch in thickness, about 

 T £o of an inch in diameter, and an inch in length, placed it upright 

 in a small trough of water under the microscope and examined it 

 with an inch objective. I found that no light passed through the 

 lumen of the tube, but that the section of the wall of the tube 

 appeared brightly illuminated. I next placed a few fine glass threads, 

 drawn from a glass rod, in the interior of the tube ; these were as 

 nearly as possible the same length as the tube and measured 10 1 00 

 of an inch in diameter. The upper end of each of these rods 

 appeared as a brightly illuminated disk in the dark field ; when the 

 focus of the microscope was altered the disk enlarged, showing that 

 the rays left the rod in a divergent direction ; in some cases when the 

 ends of the rods lay beyond the focus of the microscope, the disks of 

 light exhibited grey rings, the result of interference. 



When the lower ends of these rods were lenticular, or fused into a 

 drop, or drawn into a cone, the phenomena were the same, and in all 

 cases the action of an oblique pencil, even when the obliquity was 

 very slight, was feeble as compared with that of a pencil having the 

 direction of the axis of the rod. 



These results are such as would be predicted on the undulatory 

 theory ; all the light passing into the rod, except very oblique rays, 

 would be totally reflected, without any change of phase in the un- 

 dulations, at the surface of the glass, whilst all except the axial rays 

 would be very much enfeebled by numerous reflections and inter- 

 ference from the different lengths of the paths of the rays. I think 

 threads of a highly refractive character immersed in a medium of a 

 less refractive index, when less than -g-^o of an inch in diameter, would 

 destroy the effect of rays of only very small obliquity by interference. 



In order to determine the effect of the pigment, I covered the 

 exterior of some glass rods of of an inch in diameter with 



black varnish, and I then found it impossible to transmit any rays of 

 even the slightest obliquity through half an inch of such a rod. 



From these facts I think it may be concluded that it is probable 

 that the highly refractive structures may be regarded in the light of 



VOL. XXVII. T 



