'dchen exposed to Polar i-.ed Light. 197 



system, which quickly disappears, and is succeeded by the 

 black-centred system. This phaenomenon is seen best near -t.5° 

 of azimuth. 



When the plates or films are too thick to give the coloured 

 rings, the phaenomena 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 example, the pencil or image of a small aperture or 

 luminous body will be green, while that reflected from the first 

 surface, though in reality colourless, will appear ;tY/, 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 inter- 

 esting phaenomena, varying with the colours of the glasses, as 

 if the colours were proiluced by the absorption of polarized 

 light. 



In order to convey a general idea of the different classes 

 ol [)haenomena described in the preceding paper, I have re- 

 presented 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 

 tlisappear at 53° 11', the polarizing angle of the water, and 

 also in the various azimuths where the two interfering pencils 

 are polarized in planes at right angles to each other. At all 

 azimuths greater than these, and at angles of incidence above 

 the polarizing angle, the 'iSohitc-centred rings afipear; and at 

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

 at which the white-centred rings are seen), the black-centred 

 ■ rings appear. 



The following Table shows the values o\' x, or the azimuths 

 of disappearance of the rings, as computed from the formula 

 in p. 49 : — 



Angles of incidence. 



