14 6 Dr. Brewster on the action oj 
was thus induced to question the accuracy of Malus’s views, 
and to repeat the experiments upon which he had founded 
them. The results of this investigation, while they have 
overturned the opinions hitherto adopted, have at the same 
time led to the establishment of several points both of theore- 
tical and practical importance. 
In giving an account of these results, I shall first consider 
the effects produced upon transmitted light by a change in 
the mechanical condition of the surfaces of crystals, and then 
establish the laws according to which the interior forces affect 
and modify the forces which produce partial reflection. 
Sect. I. On the ejfiects produced upon transmitted light , by a 
change in the mechanical condition of the surfaces of crystals. 
If we take a hexaedral prism of nitrate of potash, and ob- 
serve a luminous object through two of its inclined surfaces 
that have a good natural or artificial polish, we shall perceive 
two distinct and perfectly formed images. If we now roughen 
these two surfaces, and cement upon each of them a plate of 
glass by means of balsam of capivi, the character of the two 
images will be greatly changed. The image that has suffered 
the greatest refraction will be as distinct as before, but the 
other image will be either of a faint reddish colour, or 
wholly invisible, according to the degree of roughness induced 
upon the refracting surfaces. When oil of cassia is used 
instead of the balsam, the least refracted image, if it was 
visible before, will now be completely extinguished. 
By substituting pure alcohol , or the white of an egg, instead 
of the balsam, the least refracted image will become distinct, 
and the most refracted image will be either a mass of nebulous 
