ii Proceedings. \October 5th, i8gy. 



papers might have a deterrent effect upon the supply, which had 

 hitherto been voluntary, and desiring that Mr. Faraday should 

 approach Professor Suess independently on the subject. 

 The following paper was read : — 



"A mechanical device for the solution of problems 

 in refraction and polarization." By Thomas Thorp. 



In determining the relation existing between the angles of 

 incidence and refraction, the experimental apparatus usually 

 employed, or at least the simplest form of it, has the character- 

 istics shown on Fig. /, in which AB 



F 



Fig. 1. 

 is the direction of a ray of light in one medium, say air, incident 

 upon the surface of a denser medium C, say water, contain'ed in 

 a semi-cylindrical vessel, at B, where it is refracted, and then 

 proceeds to D. 



By making AB = BD = i and putting a the angle of incidence 

 and /3 that of refraction, the lines AE and DF are the sines of a 

 and j8 respectively. 



Now these sines are found to bear the same ratio to each 

 other at all angles of the incident ray from the vertical, — different 

 media giving different ratios, which are known as refractive 

 indices. When the angle of incidence is 90° that of refraction 

 will necessarily be less than 90"", and is termed the critical angle. 



From experiments made with this form of apparatus, 

 however, very little, if any, insight can be obtained into the actual 

 working of the law of refraction as determined by it. B.ecourse 

 must therefore be had to other methods. 



