Dr. Brewster on the laws of polarisation , &c. 273 
supposing them to be perfectly represented by your general law, it will follow that 
the tint exhibited depends not on the difference of refractive densities in the direction 
of the ray transmitted, but on the greatest difference of refractive densities in 
directions perpendicular to that of the ray. These two conditions lead to the 
same result, where the effect of one axis only is considered, but they vary materially 
where two axes are supposed to be combined ; and I do not immediately perceive by 
what modification it will be possible to accommodate the laws of interference to 
these experiments. There can be little doubt that the direction of the polarisation, 
in such cases, must be determined by that of the greatest and least of the refractive 
densities in question f ; and it seems to be very possible to apply your mode of cal- 
culation to many other phenomena, in which the polarising powers of different 
crystals are combined. 
Believe me, dear Sir, 
Your faithful and obedient servant, 
Thomas Young.” 
f This supposition of Dr. Young’s is quite correct. In another paper, which will 
soon be submitted to the Royal Society, I have given a general method of finding 
the direction of the polarisation for any combination of axes. 
From the Press of 
W. BULMER & Co. 
Cleveland-row , St. James’s, 
London. 
