Dr. Brewster on the communication , &c. 157 
satisfactory answer to the questions which have been stated, 
but to communicate to glass, and many other substances, by 
the mere pressure of the hand, all the properties of the 
different classes of doubly refracting crystals. The method 
of producing these effects, and the consequences to which it 
leads, will be briefly explained in the following letter. 
SECT. I. On the communication of double refraction to glass , 
muriate of soda , and other hard solids . 
Proposition I. 
If the edges of a plate of glass , which has no action upon polarised 
light , are pressed together or dilated by any kind of force, it 
will exhibit distinct neutral and depolarising axes like all 
doubly refractvig crystals, and will separate polarised light 
into its complementary colours. The neutral axes are parallel 
and perpendicular to the direction in which the force is applied, 
and the depolarising axes are inclined to these at angles of 45 0 . 
I took a plate of glass about 1 inch broad, inches long, 
and 0.28 of an inch thick, and having compressed its edges 
by the force of screws, I found that it polarised a white of the 
first order in every part of its breadth. The depolarising 
axes formed an angle of 45 0 with the edges of the plate. By 
increasing the compressing force, it polarised a faint yellow 
light of the first order, which gradually rose into orange. 
When the screw pressed upon the glass only at a single 
point, an appearance was exhibited similar to that shown in 
Fig. 1, (PL IX.) where AB is a cubical piece of glass pressed 
in the clamp CDE by means of the screw S. Between the points 
of pressure m, n, fringes mon, mpn , are developed. Between 
