Theory of a Contractile ^Tltlier to Optical Problems. 539 



To this explanation the objection * has been made that the 

 phenomena resemble those shown by thin films, and that the 

 reflected light ought to be coloured. We may reply to this 

 that the film is comparable with the thickness of the black 

 film in Newton's rings, so that the colour shown, if any, 

 would be that of the blue of the first order, and would probably 

 — the light being very faint — hardly be noticed as colour by 

 an observer who was not specially directing his attention to 

 that point. I hope, however, shortly to investigate this 

 question by direct experiment. Such observations of a 

 preliminary character as I have made have shown a bluish 

 tint in the reflected light. Moreover, if this light appear 

 blue, so, too, ought the light which is reflected in con- 

 siderable quantity from the black spot in a soap-film. In 

 fact, if we take Remold and Rucker's value for the thickness 

 of the black spot in a soap-film, the quantity of light reflected 

 from it is much greater than that observed by Jamin near the 

 polarizing-angle. For, taking the case of normal incidence, 

 we have for the intensity of the reflected light in terms of the 

 incident the value 



1 = 



If we put jx = |, 



ft-l_ 1 



mi -,11 ^TT 2 D 2 10 D 2 



Then, approximately, 1= -^- x ^ r = -^ ~ z . 

 Now, according to Reinold and Rucker, 



••• I =3^ro=-° 013 - 



Thus over one thousandth part of the incident light ought 

 theoretically to be reflected,, and the colour of this ought to be 

 mixed in the inverse ratio of the square of the wave-lengths. 

 I am not aware that any careful photometric observations on 

 this light have been made ; at any rate it is, I think, nowhere 

 stated that it shows colour. 



Now as to the Jamin effect f, we have 

 I=M*tan*(0-0') 



* Hon. J. W. Strutt [Lord Kayleigh], " On the Reflexion of Liirht from 

 Transparent Matter," Phil. Mag. 1871. 



f Jamin, Annates de Chimie et de Physique, 3 s6r. t. xxix. Cours de Phy- 

 sique, iii. p. 525. 



