WHEN EXPOSED TO POLARIZED LIGHT. 57 



When the plates or films are too thick to give the coloured rings, the phenomena 

 of the differently polarized pencils may be finely seen by using coloured glasses, in which 

 the pencils reflected from both surfaces may be observed. If the glass is green, for ex- 

 ample, the pencil or image of a small aperture or luminous body will h^ green, while 

 that reflected from the first surface, though in reality colourless, will appear red, 

 from the physiological action of the green light upon the retina. Hence the two 

 differently polarized pencils will have different colours, as if they were the tints of 

 polarized light. If these coloured glasses are laid upon, or cemented on one side to, 

 metals or highly refracting substances, the polarization of the coloured pencils which 

 they reflect will be modified according to the principles already explained, and they 

 will exhibit many interesting phenomena, varying with the colours of the glasses, as 

 if the colours were produced by the absorption of polarized light. 



In order to convey a general idea of the diff*erent classes of phenomena described 

 in the preceding paper, I have represented two of the most important in figs. 4 and 5. 



1. Glass and Water. — When a film of aqueous vapour is laid upon glass whose 

 index of refraction is 1*508, the rings disappear at 53° 1 1', the polarizing angle of the 

 water, and also in the various azimuths where the two interfering pencils are polar- 

 ized in planes at right angles to each other. At all azimuths greater than these, and 

 at angles of incidence above the polarizing angle, the white-centred rings appear ; 

 and at all azimuths less than these, and at all incidences (except those at which the 

 white-centred rings are seen), the 6/acA:-centred rings appear. 



The following Table shows the values of x, or the azimuths of disappearance of the 

 rings, as computed from the formula in p. 49 : — 



If we now conceive A B, fig. 4, to be the section of the plane of incidence, having 

 the different incidences marked upon it from 90° to 53° 1 1', and if round a centre in 

 A B prolonged, where 0° of incidence falls, we describe the azimuthal circle Z A Z, 



MDCCCXLI. I 



