268 



Mr. B. T. Lowne on the Modifications of [Mar. 28, 



luminous points, which serve as stimuli in exciting the recipient pro- 

 toplasm in which their ends are imbedded. 



The focus of the facet when this is lenticular, in all the insects 

 which I have examined, is situated considerably deeper than the outer 

 end of the rhabdion and below the surface of the rod cells in the 

 microrhabdic eye, so that even for objects as close as of an inch to 

 the cornea, we have to deal with convergent rays and not with a focal 

 point. This indicates some mode of nerve stimulation other than the 

 union of homocentric pencils, in a point beneath the compound 

 cornea in relation with the recipient elements. Considering the small 

 size of the parts, I think it quite possible that we must look to the 

 phenomena of interference for the explanation ; at least, they must 

 play an important part in the phenomenon. 



Whatever may be the manner in which vision is accomplished the 

 size of the corneal facets and the general curvature of the cornea 

 renders the theory of J. Muller highly probable. It is true that 

 Claperede has expressed the reverse opinion, but I shall endeavour to 

 show that he has done so on insufficient data. According to his cal- 

 culation, a bee should be unable to distinguish objects of less than 

 eight inches in diameter at a distance of twenty feet from it. This 

 calculation is based on the idea that the acuity of vision in this insect 

 is the same in all parts of the field of vision, and that the general 

 surface of the common cornea is approximately a segment of a sphere. 

 This is not the case, for the angles subtended by the adjacent facets 

 in the centre of the cornea, which is considerably flattened, is not 

 more than half a degree at the most ; so that on J. Muller' s theory, 

 supposing each facet to give rise to only a single luminous impression, 

 the bee should be able to distinguish objects of about two inches in 

 diameter at a distance of twenty feet, an acuity of vision quite equal 

 to account for all the phenomena of vision in bees, 



I have measured the curvature of the cornea of a number of 

 insects, with a view to determining the angles made by the lines of 

 vision drawn from the centre of adjacent facets. This is done in the 

 following manner : — A magnified image of the cornea is thrown on a 

 sheet of white paper, by means of a microscope and camera lucida, 

 and the curve of its profile drawn; in this way I have found the 

 principal meridians. These curves approach more or less closely to an 

 epicycloid. 



It is easy with such curves and the size of the corneal facets to 

 determine the angles made by adjacent facets. The angles vary 

 inversely as the radius of curvature, and, therefore, the acuity of 

 vision varies directly as the radius of curvature when the diameter of 

 the facets remains the same, and inversely as the diameter of the 

 facets when these vary in size. In many insects, as Tab anus ^ the 

 peripheral facets of the cornea are twice or three times the diameter 



