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of these two bodies is nearly the same on the less refrangible rays : 

 hence it may happen that a larger proportion of the former than of 

 the latter is transmitted, and the pencil formed by reflexion will 

 then appear blue. The partial decomposition thus effected in the 

 incident rays will be the same in kind, though it may vary in degree, 

 at different angles of incidence, and cannot therefore give rise to any 

 variation of colour in the reflected rays, although they may differ in 

 intensity according to the obliquity of the incidence. By using dif- 

 ferent kinds of glass, and of interposed fluids, the author obtained 

 various analogous results, different rays of the spectrum being sepa- 

 rated according to the prevalence, in each particular case, of one or 

 other of the opposite actions exerted upon them by the solid and the 

 fluid medium. The author directed his attention more particularly 

 to those conditions in which the nearest approach could be made to 

 a perfect equilibrium of all the forces which affect the incident rays. 

 The solids which he employed in his experiments were two prisms of 

 plate glass, of which the sections were right-angled isosceles triangles, 

 and differing but very slightly in their refractive indices. The fluids 

 were castor oil and balsam of copivi, the former having a less, and the 

 latter a greater refractive power than the glass prisms ; a thin film of 

 either fluid being interposed between them. With castor oil, and 

 within the limit of total reflexion, the reflected light is yellow ; on 

 gradually diminishing the angle of incidence, it passes in succession 

 through all the tints of three orders of colours, of which the details 

 are presented in a table exhibiting those which correspond to different 

 angles of incidence. When the incident light is homogeneous, no 

 colours are seen, but the reflected pencils have their maxima and 

 minima of intensity ; like the rays of thin plates, or the fringes of 

 inflected light formed by homogeneous rays. When copivi balsam 

 is employed as the fluid medium, the same orders of colours are ob- 

 tained by reflexion, but at smaller angles of incidence than with cas- 

 tor oil. 



Having ascertained that at a temperature of about 94 the mean 

 refractive index of the balsam became equal to that of the glass 

 prisms, the author examined the influence of a gradual elevation of 

 temperature upon the colours of the reflected pencils ; and found that 

 no particular change marked the instant when the refractive densi- 

 ties of the two media became equal ; although when the temperature 

 was increased considerably, the tints entirely disappeared. Analogous 

 results were obtained by employing prisms of obsidian instead of 

 glass. 



The author next engaged in more extensive series of experiments 

 with various fluids interposed between glass prisms ; and states their 

 results in the form of a table, showing more especially the periods of 

 colours produced at the separating surfaces by the different kinds of 

 oils. He considers the facts which are there detailed, as establishing 

 the existence of reflecting forces at the confines of media of the same 

 refracting power ; and as proving, first, that the reflective and refrac- 

 tive forces in these media do not follow the same law : and, secondly, 



