194 DE. BREWSTER ON REFLECTED LIGHT. 



o o m is ten or twelve times more luminous tlian psn; whereas at smaller inci- 

 dences than 61° 54', the pencil psn surpasses oqm'vn the intensity of its light. 

 By the appHcation of heat psn becomes yellowish- white, and increases greatly 

 in intensity. It now approaches at oblique incidences to the brightness of o^; m, 

 but is still inferior to it, while at small incidences it surpasses it in intensity. 



In the preceding experiments the solid had nearly the same refractive den- 

 sity as the balsam. We shall now take a solid, namely obsidian, which has 

 nearly the same i-efractive power as the oil. 



When the lower prism B is obsidian, and the film C D, H G balsam of capivi, 



the ray psn passes through three orders of colours ; namely, 



rWhite, 



^ ^ Yellow, 



1st Order. < _, , 

 Red, 



[.Limit of red and blue at 73°. 



-Blue, 



Bluish-green, 



2nd Order.< Yellowish white. 



Reddish white, 



LPink, faint. 



r Bluish, 



' 1 Bluish-white. 



These colours are by no means good, nor are they much improved by heat, 



which approximates the refractive power of the fluid to that of the solid. The 



heat reduces the orders to two, each colour being now developed at a much 



smaller angle of incidence. The first order, for example, which ended at an 



incidence of 73°, now ends at an incidence of 52°. When the heat is so great 



that we cannot touch the prisms with the hand, all the colours are effaced. 



If we now substitute the castor oil in place of the balsam, no colours are 



visible ; but the reflected pencil psn is white and bright, notwithstanding the 



coincidence between the refractive energies of the solid and the fluid. Heat 



increases the intensity of the pencil, but produces no colour. 



Hitherto we have considered the action of the two surfaces of the film as 



exhibited separately in the two images displaced laterally by the prismatic 



shape of the fluid. We shall now briefly notice the phsenomena which are pre- 



