DR. BREWSTER ON REFLECTED LIGHT. 205 



from the glass ; and even if it did, some one of the oils with which it was in 

 contact in the foregoing experiments must have had the same refractive energy, 

 and must thus have deprived it of its power to develope the periodical tints. 

 In the hope of unravelling this mystery, I took t^vo prisms of glass cut out of 

 the same plate, and which gave fine periodical colours with castor oil. By the 

 aid of screws I pressed the bases of the prisms into optical contact : at great 

 incidences the light was yellow ; and by diminishing the inclination of the ray 

 it became gradually orange and deep red when it vanished, no light being 

 visible at smaller angles of incidence. In this experiment the surfaces of the 

 two films, if they do exist, were brought into optical contact, so that we ought 

 to have had orders of colours corresponding to a film of twice the thickness. '■ 



But even if such a film could be supposed to exist invisibly on the glass, it 

 could not afford any explanation of the splendid colours which are exhibited 

 when the solid is a ci-ystallized mineral, and where its tint is related to its axis 

 of double refraction. That some unrecognised physical principle is the cause 

 of all these pheenomena, will appear still more probable when I submit to the 

 Society a paper on the very same periods of colour produced at similar angles 

 of incidence, by the surfaces of metals and transparent solids when acting 

 singly upon light. 



The action of the surfaces of crystallized bodies presents many remarkable 

 phsenomena, in the investigation of which I have been long occupied. The 

 results to which I have been led will form the subject of two communications. 

 The first will treat of the action of the surfaces of bodies as an universal 

 mineralogical character, with the description of a lithoscope for discriminating 

 minerals. The second will contain an inquiry into the influence of the doubly 

 refracting forces upon the ordinary forceti which reflect and polarise light at 

 the surfaces of bodies. My early experiments on this subject are recorded in 

 the Phil. Trans, for 1819, but I have resumed the inquiry, and have obtained 

 results of considerable interest. 



*9mi38 



^-'^"' Jllerly, February 2ml, 1829. " 



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