Royal Society. 381 



in intensity according to the obliquity of the incidence. By using 

 different 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 aft'ect 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 diff"ering but very slightly in their refractive indices. The fluids 

 were castor oil and balsam of capivi, 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 reflection, the reflected light is yellow j 

 on gradually diminishing the angle of incidence, it passes in succes- 

 sion through all the tints of three orders of colours, of which the de- 

 tails are presented in a table, exhibiting those which correspond to 

 different angles of incidence. When the incident light is homoge- 

 neous, 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 capivi 

 balsam is employed as the fluid medium, the same orders of colours 

 are obtained by reflection, but at smaller angles of incidence than 

 with castor oil. 



Having ascertained that, at a temperature of about 94 degrees, 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 inflected pencils, and found that 

 no particular change marked the instant when the refractive density 

 of the two media became equal, although, when the temperature was 

 increased considerably, the tints entirely disappeared. Analogous re- 

 sults 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 diff'erent kinds of 

 oils. He considers the facts which are there detailed as establisliing 

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

 refracting power, and as proving, Jst, that the reflective and refract- 

 ing forces in these media do not follow the same law j and, 2dly, that 

 the force which produces reflection varies according to a diff'erent law 

 in different bodies. The reflective forces of the solid and the fluid 

 may be conceived to decrease in various ways : 1st, they may re- 

 sj)ectively extend to different distances from the reflecting surface, 

 and decrease according to the same law ; 2{ily, they may extend to 

 different distances, and vary according to a diff'erent law ; and, lastly, 

 they may extend to the same distance, and vary according to different 

 laws. Whether the refracting forc(s follow the same law in solids and 

 in fluids, it is extremely difficult to determine by direct experiment ; 



but 



