18 Mr. K. A. Houstoun on the Effect of a 



The first observations were made on a very old prism, the 

 surface-films of which must have grown through age. The 

 incident light was polarized at an angle of 45° to the plane 

 of incidence. 



(j>, the angle of incidence, was calculated from R, the angle 

 between the directions of the telescope and collimator, and n 

 the index of refraction, by the following formula : 



R=a + /3 + sin -1 [n sin {(f) — a,}) +sin _1 (w sin {<£ — /3}). 



Observations were made for a number of angles, starting 

 from the limiting angle of total reflexion. After some unsuc- 

 cessful attempts, two good series of observations were obtained, 

 — ten different angles and ten observations to each angle. 

 Before the experiment the prism was left all night in alcohol, 

 then washed thoroughly with water and dried with a linen 

 cloth. It was placed in such a position upon the spectrometer- 

 table, that when it was rotated the light always fell upon the 

 same part of the hypotenuse surface during the same series 

 of observations. The two series of observations given were 

 made for light that was totally reflected on different parts of 

 the surface. 



A is the observed phase-difference. In the next column to A 

 stands the mean error to give an idea of the accuracy of the 

 observations. A is the phase-difference calculated according 

 to Fresnel's formula. 



FresnePs formula is 



sin 2 <£ = 



taniA = — " 



sm <p tan 9 



The formula given by Drude is 



e 1 



tan ^A / = tsaiiA — 



n sin cf> tan <f> 



