Surface-Film in Total Reflexion. 



21 



and telescope B remained 

 fixed and opposite to one 

 another during the whole 

 experiment. According to 

 the theory, the phase -dif- 

 ferences produced "by the 

 surface-film at R and V, 

 and at S and T cancel 

 one another, The index of 

 refraction of the glass was 

 determined by the method 

 of minimum deviation for 

 each of the four acute 

 angles. The result was 

 1-5195 + 2. The sides were 

 not exactly parallel, the 

 greatest error being as much 

 as 5'. To simplify the cal- 

 culation, the rhombs were 

 considered perfect, the angles 

 being 54° 40' and 125° 20'. 

 To avoid the possibility of 

 error from this approxima- 

 tion, no measurements were 

 made on the first part of the 

 curve, where the phase-dif- 

 ference increases rapidly 

 with the angle of incidence. 

 The rhombs were placed 

 on a platform that slid in 

 rails on the spectrometer- 

 table. The table could be 

 rotated relatively to the tele- 

 scopes, and its position read 

 on a scale. The platform 

 was part of the original 

 equipment of the instru- 

 ment. To determine the 

 angle of incidence and place 

 the rhombs symmetrically, 

 the auxiliary telescope G was 

 used. C remained always 

 fixed, and the angle between 

 A and C was determined 

 by fitting C with a Gauss ocular, and moving B until the 

 cross-wires in C coincided with its cross-wires. The angle 



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