DR. BREWSTER ON REFLECTED LIGHT. 
189 
In the year 1814, when I was investigating the law of polarisation for light 
reflected at the separating surface of different media*, I had occasion to in- 
close oil of cassia between two flint glass prisms. The blue colour of the re- 
flected light at first surprised me ; but though the fact was new, and the ex- 
periment itself interesting, the decomposition of the light was obviously expli- 
cable upon known principles. Although the refractive 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 ana- 
logous results, in which different rays of the spectrum 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 extinguished, the colour of the reflected pencil has a yellow tinge ; and it is 
obvious that the resulting pencil can never have a decided colour, but must 
always be bluish or yellowish. 
As the indices of refraction remain the same for all obliquities of incidence, 
the tint of the reflected pencil, though it varies in intensity, can never vary in 
its colour ; so that we cannot 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 pencil must necessarily have a different 
tint from the incident pencil, excepting in the extreme case where the two bodies 
in contact have mathematically the same refractive and dispersive powers. 
I was now anxious to observe the effect of an approximation to this last 
condition, or to a perfect equilibrium of all the forces which affect the incident 
rays ; as it is often in extreme cases, and at a limit such as this, that nature 
delights in the development of new phaenomena. This experiment, however, 
was attended with more difficulty than I expected ; but amid the numerous 
* Phil. Trans. 1825, p. 137. 
