1062 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



Cylinders which act as Lenses, and give an Optical Image.* — 

 Prof. S. Exner having previously found,f in examining tho eye of 

 Hydrophilus pieeus with his micro-refractometer, that each facet of tho 

 cornea is a cylinder of continually increasing refractive power from 

 the circumferenco to the axis, has been led to study the optical action 

 of such a cylindor. That tho images produced by the facets are not 

 only duo to their spherical terminations was shown by removing the 



Fig. 233. 



isEEfiSB 



latter, when an inverted image was produced by the section between 

 ab,'cd (fig. 233), and this must be due to the fact that each cylinder 

 acts as a lens by reason of its varying optical density. The subject 

 is mathematically treated by Prof. Karl Exner, but the general results 

 are suggested by the following simple considerations : — 



Let x y (fig. 234) be the axis of the cylinder of which ac,bd, are 



Fio. 234. 



the plane terminations perpendicular to x y, and suppose the refractive 

 index to be a maximum along the axis, and to diminish regularly 

 towards the circumference. Then the course of a ray x m incident 

 at m is shown by the curved line x m y, since the ray on encountering 

 successive strata of varying density will be continually refracted 



* Ffliiger's Arch. 

 (10 figs.). 



f. d. 



gesammt. Fhysiol., xxxviii. (1886) pp. 274-90 

 t See this Journal, ante, p. 328. 



