light at the surfaces of different media. 211 



equal refractive powers explains many curious phenomena, 

 &c."* 



In the year 1814, when I was investigating the law of po- 

 larisation for light reflected at the separating surface of differ- 

 ent media-(-, I had occasion to inclose oil of cassia between two 

 flint glass prisms. The blue colour of the reflected light at 

 first surprised me ; but though the fact was new, and the ex- 

 periment itself interesting, the decomposition of the light was 

 obviously explicable upon known principles. Although the re- 

 fractive density of oil of cassia exceeds greatly that of flint 

 glass for the mean rays, yet the action of the two bodies on 

 the less refrangible rays is nearly the same ; and hence the red 

 rays must be in a great measure transmitted, while there will 

 be reflected a small portion of the orange, a greater portion of 

 the yellow, a still greater proportion of the green, and a very 

 great proportion of the blue : and consequently the colour of 

 the pencil formed by reflection must necessarily be principally 

 blue. 



By using different kinds of glass and different oils I obtained 

 various analogous results, in which different rays of the spec- 

 trum were extinguished by effecting (as far as possible) an 

 equilibrium between the two opposite actions exerted upon them 

 by the solid and the fluid media. When the blue light is ex- 

 tinguished, the colour of the reflected pencil has a yellow tinge; 

 and it is obvious that the resulting pencil can never have a de- 

 cided colour, but must always be bluish or yellowish. 



As the indices of refraction remain the same for all obliqui- 

 ties of incidence, the tint of the reflected pencil, though it va- 

 ries in intensity, can never vary in its colour ; so that we can- 

 not obtain any succession of tints or coloured rings from this 

 partial decomposition of the incident rays. 



These observations establish it as a general fact, that in all 

 cases of reflection from transparent surfaces, the reflected pen- 

 cil must necessarily have a different tint from the incident pen- 

 cil, excepting in the extreme case where the two bodies in con- 

 tact have mathematically the same refractive and dispersive 

 powers. 



* Treatise on Light y § 547, 548. 

 t Phil. Trans. 1825, p. 137. 



