177 
refraction on the glass s &c. by pressure. 
which we must multiply the thickness of any plate of j e hy> ,a 
order to obtain the thickness of a thin plate which would afford 
by reflection a tint similar to its maximum tint, is ■g r ^4* 
The following are the constant factors for different doubly 
refracting substances. 
Calcareous spar, 
Rock crystal. 
i 
TT 
__L_ 
3 6 0 
3 6 0 
^according to Biot. 
4TO 
Sulphate of lime. 
Mica, 
Isinglass 
Beryl, - 
Glass, - - - 
If the isinglass were made capable of resisting a higher de- 
gree of distension it would give a constant factor, approaching 
still nearer to that of mica. 
6T4 
yi— according to Biot. 
E_ 
i 218 O 
Upon reviewing the general principles contained in the 
preceding Propositions, I cannot but allow myself to hope 
that they will be considered as affording a direct solution of 
the most important part of the Problem of double refraction. 
The mechanical condition of both classes of doubly refracting 
crystals, and the method of communicating to uncrystallized 
bodies the optical properties of either class, have been dis- 
tinctly ascertained, and the only phenomenon which remains 
unaccounted for, is the division of the incident light into two 
oppositely polarised pencils. How far this part of the sub- 
ject will come within the pale of experimental inquiry, I do 
not presume to determine ; but without wishing to damp that 
ardour of research which has been so happily directed towaias 
this branch of optics, I fear that, as in the case of electrical 
and magnetical polarity., we must remain satisfied with refer- 
mdcccxvi, A a 
