228 Dr Brewster on reflected light. 



but the new face gave only one colour, which was a bright 

 blue, but which, from the nature of the surface, I could not 

 trace to high or low incidences. 



As these results seemed to indicate that the glass had receiv- 

 ed from exposure to the air some incrustation, or had absorbed 

 to a small depth some transparent matter in a minute state of 

 division, or had suffered some change in its mechanical condi- 

 tion, I made various fruitless attempts to ascertain the nature 

 of the change. No superficial tarnish could be rendered visi- 

 ble, either by the microscope or by any other means. I boiled 

 the prisms in muriatic acid, and in strong alkaline solutions : I 

 steeped them in alcohol, and applied a strong pressure along 

 their surfaces ; but I could not in the slightest degree change 

 their action upon light. 



If a superficial film had been formed upon the glass of such 

 a thickness as to give the periodical colours, then its refractive 

 power must be different from that of the glass. I therefore took 

 a prism which gave the periodical colours, and another of the 

 same glass which had been deprived of this property ; and I 

 found that they polarised light at exactly the same angle. I 

 then placed them upon the base of a flint glass prism with oil 

 of cassia interposed, and I determined that the angle at which 

 they reflected light totally was the same *. Hence it was ma- 

 nifest that the supposed film did not differ in refractive power 

 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 two 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 be- 

 came gradually orange and deep red when it vanished, no light 

 being visible at smaller angles of incidence. In this experi- 

 ment the surfaces of the two films, if they do exist, were 



• The prism which produced the periodical colours, did not give so dis- 

 tinct a boundary between partial and total reflection as the other. 



