194 
DR. BREWSTER ON REFLECTED LIGHT. 
1st Order. <( 
n 
1 ; 
o q m is ten or twelve times more luminous than p s?i; whereas at smaller inci- 
dences than 61° 54 f , the pencil p sn surpasses o q m in the intensity of its light. 
By the application of heat p s n becomes yellowish-white, and increases greatly 
in intensity. It now approaches at oblique incidences to the brightness of op 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 refractive power as the oil. 
When the lower prism B is obsidian, and the film C D, H G balsam of capivi, 
the ray p s n passes through three orders of colours ; namely, 
'White, 
Yellow, 
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 p sn 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 phenomena which are pre- 
3rd Order. 
