Surface-Film in Total Reflexion. 



23 



In both the above cases the film was the natural surface- 

 film, and not a homogeneous layer due to polishing, as the 

 glass surfaces had not been polished for a very long time. 



An attempt was then made to verify the theory further by 

 depositing very thin artificial surface-films of collodion on the 

 hypotenuse surface of a prism — which had previously shown no 

 trace of such an effect. The artificial film should produce the 

 same effect as the natural films. The films were made after 

 the manner of Wiener, by putting two or three drops of a 

 very dilute solution of collodion in ether and alcohol on the 

 hypotenuse surface of the prism, pressing them down with a 

 similar surface, until they filled all the space between, and 

 then drawing the two surfaces apart. The ether and alcohol 

 then evaporated, leaving a homogeneous skin of collodion. 



The prism used was right-angled and isosceles, and the index 

 of refraction for the D-lines was 1/513. The refractive index 

 of collodion is greater than this; and so we had a uniform 

 layer, the index of refraction of which did not lie between 

 that of the glass and that of the air, and which gave an effect 

 in the opposite direction to that produced by the natural film. 

 The film was kept dry during the observations by placing a 

 quantity of calcium chloride on the spectrometer-table. The 

 presence of dampness was easily detected; it lowered the index 

 of refraction of the film to below that of the glass, and we 

 had a deviation from FresneFs formula in the other direction. 



Some trouble was experienced in getting a film of a suitable 

 thickness ; if the film is too thin, the effect is comparable with 

 the errors of measurement, and the formula does not hold when 

 it is too thick. The set of observations given, however, 

 shows the effect of the film very well. 



Collodion Film. 

 Limiting angle of total reflexion, = 41° 22' 



<p. 



B. 



\- 



A. 



Mean 

 Error. 



A -A. 



A'. 



1 

 A r -• A. 



o ' 



o , 



o 



o 



o 



o 



o 



o 



42 30 



82 26 



25-4 



22 - 2 



o-i 



32 



21-6 



-0-6 



44 



86 58 



35-3 



30 7 



0-3 



3-6 



31-7 



4-1-0 



45 30 



91 31 



40-5 



37-2 



04 



3-3 



37-0 



-0-2 



47 



96 2 



435 



40-3 



02 



3-2 



40-2 



-01 



48 30 



100 36 



45-2 



42-4 



02 



2-8 



42-2 



-0-2 



50 



105 10 



45-9 



43-5 



01 



2-4 



43-1 



-0-4 



51 30 



109 44 



46-1 



43-4 



0-2 



2-7 



43-5 



+04 



53 



114 18 



45-8 



43-2 



01 



2-6 



43-4 



+0-2 



54 30 



118 54 



45-2 



430 



0-3 



2-2 



42-9 



-o-i 



56 



123 28 



443 



424 



0-3 



21 



422 



+0-1 



:0029. 



</>, E, A , A, and A' have the same meaning as before. 



