DR. BREWSTER ON REFLECTED LIGHT. 
203 
ference of two portions of light cannot be questioned ; though it does not 
appear how these interfering pencils are generated. If we adopt the hypothesis 
of the reflecting forces shown in Fig. 4, we may conceive the light reflected 
about C D to be interfered with by the light reflected about C' D', so that the 
same effect nearly might be produced as if C D, C' D' were the limits of a thin 
plate. If this supposition is not admissible, we may hazard the conjecture, 
countenanced by some facts which will presently be stated, that an invisible 
film differing in refractive power from the plate glass, has been formed upon 
its surface. 
There is one phenomenon which has been more than once mentioned, $nd 
which requires some further notice ; namely, the decrease in the intensity of 
the pencil as the incidence becomes more oblique. In re-examining this very 
perplexing fact, which takes place in the prism B though it does not produce 
periodical colours, I have observed at a great incidence a distinct change of 
colour, from a bluish gray to a blue ; so that I have no doubt that in this case 
the tints are those of a long period approaching slowly to its minimum. This 
consideration led me to suppose that in the case of balsam of capivi and other 
fluids, where the first order ends at and below 65°, there might be another 
minimum between that angle and 90°, which was prevented from showing 
itself by the intensity of the reflected light. This conjecture was confirmed by 
a careful repetition of the experiment with cubes of glass, and also by another 
prism in which the only tint was a pink red at an incidence of about 85°, 
and a blue shading off into a greenish gray at less angles of incidence. In 
this case, then, there was only one minimum at about 85°. A slight diminu- 
tion of temperature shifted this minimum towards 90°, while an increase of 
temperature brought it to a lesser incidence than 85°. 
Although there can be little doubt that periodical tints are more or less 
developed in every combination of solids and fluids of the same refractive 
power, yet their production in combinations where there is much uncompen- 
sated refraction, is influenced by certain changes on the surface of the solid, 
the nature and origin of which I have in vain attempted to discover. 
Having observed that the colours occasionally became less bright after the 
media had remained some time in contact, and that different parts of the same 
surface produced the same tint at inclinations sensibly different, I took a prism 
which gave with castor oil three fine periods ; and having brought it to a white 
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