4G Mr. A. Whitwell on Refraction 



junctions are in use together, it is better to have a hirger 

 selector-switch with multiple ways, instead of using the 

 tour-way one provided on the potentiometer proper. 



All terminals of the potentiometer are arranged in one row 

 at the back, and a coA'er over the box is provided, which, 

 however, does not come over the terminals, so that the wires 

 need not be detached when the instrument is not in use. 



I am indebted for suggestions and help to Mr. Keeling, 

 and the making and adjustment of the coils of the instru- 

 ment has been mainly done by Mr. Melsom, while the rest of 

 the constructional woik has been carried out very efficiently 

 by the mechanic to the physical department, Mr. Murfitt. 

 "^Teddingtoii, Feb. 1P03. 



VI. On Refraction at a Cylindrical Surface. 

 By Arthur Whitwell, M,A., A.R.C.Sc.I* 



[Plate m.] 



rilHE object of this paper is to describe and illustrate the 

 JL position and form of the focal areas produced by the 

 refraction at a cylindrical surface, bounding two media of 

 different refractive indices, of light diverging from or con- 

 verging to a point. 



In general, when light diverging from or converging to 

 a point falls on any surface bounding two media of different 

 refractive indices, if a plane can be drawn through the point 

 to cut an element of, or the whole surface, symmetrically, 

 then all the light will really or virtually pass through a focal 

 line or focal area in this plane. It is usual, when considering 

 elements of the surface only, to use the term focal line, but 

 it should always be remembered that these focal lines are in 

 general elements of area. In the case we are about to 

 consider there are two planes, about which the surface is 

 symmetrical, which can be drawn through the radiant-point. 

 One of these planes will contain the axis of the cylinder, and 

 the other will be normal thereto. 



We will consider first the plane containing the radiant- 

 point and the axis of the cylinder. 



Let fig. 1 (PI. III.) be a plan and elevation of the cylinder 

 of radius r, and let the radiant-point o be at a distance a from 

 the axis. Draw the elevations and plans of two symmetrical 

 incident rays and of the corresponding normals and refracted 

 rays, join o c and produce to meet the refracted ray in a 

 point g. The two refracted rays will meet at the point c/, 



* Commimicated by the Physical Society : read Marcb 27, 1903. 



