DR. BREWSTER ON REFLECTED LIGHT. 
193 
3rd Order.<( 
Colours. 
Angles of Incidence 
RrE. 
on the Surface 
CoD. 
•Bluish pink .... 
O 
.... 28 . . 
0 / 
. . 63 8 
Full blue 
.... 26 . . 
. . 61 54 
Bluish green .... 
22 
. . 59 23 
Bluish yellow .... 
.... 18 . . 
. . 56 50 
Yellow ...... 
.... 10 . . 
. . 51 37 
Reddish yellow . . . 
.... 1 . . 
. . 45 40 
Red 
.... — 8 . . 
. . 39 42 
Pink red 
13 . . 
. . 36 25 
Limit of pink and blue 
.... —16 . . 
. . 34 28 
Blue 
.... —22 . . 
. . 30 37 
Bluish green .... 
.... -26 . . 
. . 28 56 
Green 
... -30 . . 
. . 25 29 
Yellowish green . . . 
.... -41 . . 
. . 19 13 
Haying ascertained that at a temperature of about 94° the mean refractive 
index of the balsam was nearly equal to that of the glass prisms, I proceeded 
to examine the influence which a varying temperature from 50° to above 94° 
exercised over the intensity and the colour of the reflected pencil. 
The prisms were therefore fixed so as to exhibit the full blue of the second 
order, and the heat was gradually applied. The colour of the tint was ob- 
viously improved by heat, though the intensity of its light was diminished. 
No particular change marked the instant when the refractive density of the 
glass and the balsam was equal. Beyond 94° the intensity of the tints in- 
creased in consequence of the diminution in the refractive power of the balsam ; 
but when the temperature was considerably augmented, the tints completely 
disappeared. 
Let us now attend to a very remarkable pheenomenon exhibited in the rela- 
tive intensities of the pencils o qm and p s n. At an angle of incidence of 61° 54' 
on the surface CoD, and at a temperature of about 50°, the pencil o qm is a full 
blue, while p s n is a grayish white of rather less intensity than the blue pencil. 
By increasing the angle of incidence, the pencil o qm increases rapidly in inten- 
sity, while the gray pencil diminishes slowly : so that at an incidence of 74° 
MDCCCXXIX. 2 C 
