light at the surfaces of different media, 227 



other fluids, where the first order ends at and below Qb°, there 

 might be another minimum between that angle and 90°, which 

 was prevented from showing itself by the intensity of the re- 

 flected light. This conjecture was confirmed by a careful re- 

 petition of the experiment with tubes of glass, and also by 

 another prism in which the only tint was a pink red at an in- 

 cidence of about 85°, and a blue shading off into a greenish 

 gray at less angles of incidence. In this case, then, there was 

 only one minimum at about 85°. A slight diminution of tem- 

 perature shifted this minimum towards 90°, while an increase 

 of temperature brought it to a lesser incidence than 85°. 



Although there can be little doubt that periodical tints are 

 more or less developed in every combination of solids and fluids 

 of the same refractive power, yet their production in combi- 

 nations where there is much uncompensated refraction, is influ- 

 enced by certain changes on the surface of the solid, the nature 

 and origin of which I have in vain attempted to discover. 



Having observed that the colours occasionally became less 

 bright after the media had remained some time in contact, and 

 that different parts of the same surface produced the same tint 

 at inclinations sensibly different, I took a prism which gave 

 with castor oil three fine periods ; and having brought it to a 

 white heat, I then ground and repolished its faces. It now 

 ceased to give the same periods as before ; but it still decom- 

 posed the white light reflected from its confines with balsam 

 of capivi, and reflected a strong pencil of a blue colour, even 

 when the opposite refractions were perfectly compensated. I 

 now ground and repolished one of the faces of the obsidian 

 already mentioned. It also ceased to give the colours with 

 balsam of capivi formerly described ; but it now produced, 

 when combined with castor oil, with which it previously gave 

 no colours, a beautiful yellow pencil, the reflected light being 

 white at great incidences, and becoming yellower as the ray 

 approached the perpendicular. In order to ascertain what 

 changes might be owing to the processes of grinding and 

 polishing, I sought out an old face of fracture in a plate of 

 glass, whose wrought surfaces gave fine periodical colours ; 

 and I formed a new face of fracture. The old face, which had 

 been exposed for ten years, gave the usual orders of colours ; 



