DR. BREWSTER ON REFLECTED LIGHT. 
205 
from the glass ; and even if it did, some one of the oils with which it was in 
contact in the foregoing experiments must have had the same refractive energy, 
and must thus have deprived it of its power to develope the periodical tints. 
In the hope of unravelling this mystery, I took two prisms of glass cut out of 
the same plate, and which gave fine periodical colours with castor oil. By the 
aid of screws I pressed the bases of the prisms into optical contact : at great 
incidences the light was yellow ; and by diminishing the inclination of the ray 
it became gradually orange and deep red when it vanished, no light being 
visible at smaller angles of incidence. In this experiment the surfaces of the 
two films, if they do exist, were brought into optical contact, so that we ought 
to have had orders of colours corresponding to a film of twice the thickness. 
But even if such a film could be supposed to exist invisibly on the glass, it 
could not afford any explanation of the splendid colours which are exhibited 
when the solid is a crystallized mineral, and where its tint is related to its axis 
of double refraction. That some unrecognised physical principle is the cause 
of all these phenomena, will appear still more probable when I submit to the 
Society a paper on the very same periods of colour produced at similar angles 
of incidence, by the surfaces of metals and transparent solids when acting 
singly upon light. 
The action of the surfaces of crystallized bodies presents many remarkable 
phenomena, in the investigation of which I have been long occupied. The 
results to which I have been led will form the subject of two communications. 
The first will treat of the action of the surfaces of bodies as an universal 
mineralogical character, with the description of a lithoscope for discriminating 
minerals. The second will contain an inquiry into the influence of the doubly 
refracting forces upon the ordinary forces which reflect and polarise light at 
the surfaces of bodies. My early experiments on this subject are recorded in 
the Phil. Trans, for 1819, but I have resumed the inquiry, and have obtained 
results of considerable interest. 
Allerly , February 2nd, 1829. 
