204 
DR. BREWSTER ON REFLECTED LIGHT. 
heat, I then ground and repolished its faces. It now ceased to give the same 
periods as before ; but it still decomposed the white light reflected from its 
confines with balsam of capivi, and reflected a strong pencil of a blue colour, 
even when the opposite refractions were perfectly compensated. I now ground 
and repolislied one of the faces of the obsidian already mentioned. It also 
ceased to give the colours with balsam of capivi formerly described ; but it 
now produced, when combined with castor oil, with which it previously gave 
no colours, a beautiful yellow pencil, the reflected light being white at great 
incidences, and becoming yellower as the ray approached the perpendicular. 
In order to ascertain what changes might be owing to the processes of grind- 
ing and polishing, I sought out an old face of fracture in a plate of glass, 
whose wrought surfaces gave fine periodical colours ; and I formed a new face 
of fracture. The old face which had been exposed for ten years gave the usual 
orders of colours ; but the new face gave only one colour, which was a bright 
blue, but which, from the nature of the surface, I could not trace to high or 
low incidences. 
As these results seemed to indicate that the glass had received from ex- 
posure to the air some incrustation, or had absorbed to a small depth some 
transparent matter in a minute state of division, or had suffered some change 
in its mechanical condition, I made various fruitless attempts to ascertain the 
nature of the change. No superficial tarnish could be rendered visible, either 
by the microscope or by any other means. I boiled the prisms in muriatic 
acid, and in strong alkaline solutions : I steeped them in alcohol, and applied 
a strong pressure along their surfaces ; but I could not in the slightest degree 
change their action upon light. 
If a superficial film had been formed upon the glass of such a thickness as 
to give the periodical colours, then its refractive power must be different from 
that of the glass. I therefore took a prism which gave the periodical colours, 
and another of the same glass which had been deprived of this property ; and I 
found that they polarised light at exactly the same angle. I then placed them 
upon the base of a flint glass prism with oil of cassia interposed, and I de- 
termined that the angle at which they reflected light totally was the same*. 
Hence it was manifest that the supposed film did not differ in refractive power 
* The prism which produced the periodical colours, did not give so distinct a boundary between 
partial and total reflection as the other. 
