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of human vision was about 15°, and thought that if it were 

 possible to increase this angle to, say, 60°, we should be able to 

 identify the flowers in the last-mentioned instance, because 

 the passing of the image across the whole retina in, say, one-sixth 

 of a second, would give the brain sufficient time to recognise the 

 flowers on the bank. 



Mr. Julius Rheinberg pointed out that in a number of pin-hole 

 and diffractive experiments made with the microscope some time 

 ago he had obtained practically similar results in the image plane, 

 w T hether a lens or objective of low power or one of high power, 

 or whether no lens at all, was used behind' the pin-hole. This 

 accorded with theory, and showed that the convexitv of the lens 

 played no special role under such circumstances. Assuming the 

 usual theory as to the insect's eye, that what corresponded to the 

 retina was at some little distance from the facets themselves, this 

 would show that the shape or convexity of the facets did not 

 matter much, and, in fact, tended to corroborate Dr. Spitta's 

 remarks as to the facets acting rather in the manner of pin-holes 

 than lenses. The usual theory, so far as he understood it, was 

 that the function of each facet was to form on the spot of the 

 retina apportioned to it the image of just the one small part of 

 the object which was directed towards the axis of the facet, all 

 the light from the other portions of the object being absorbed or 

 cut off by the pigment surrounding the "pyramids," at whose 

 lower end the surface corresponding to the retina was supposed 

 to be. Helmholtz, in his Physiological Optics, had pointed out 

 that even under these conditions the convexity of the facets 

 would serve the useful purpose of better concentrating the light 

 which falls axially on the facet, and in better helping to get rid 

 of the light which falls on it from other directions. As the 

 optical image of a point of light, whether formed by a lens 

 or a pin-hole, was a little disc which could not be less than a 

 certain size, the speaker had made a few rough calculations as 

 to their size in the case of a fly's eye, of which he had a section, 

 and found that a single disc would more than half cover, and in 

 most cases completely cover, the spot on the retina apportioned 

 to each facet, provided the fly saw T with light of the same wave- 

 length as we do. The calculation assumed that what corre- 

 sponded to the retina was at the lower end of the pyramids, for 

 if the surface on which the image was formed was just behind 



