hi file polarized Tints certain Crystals with one asois. 337 
this law of proportionality admits of exceptions ; and to the in- 
stances there adduced, I have now to add other still more re- 
markable ones, which, if I mistake not, afford abundant proof 
that it has no foundation whatever in the nature of light. In- 
deed, it may be observed, that the last sentence in the passage 
just quoted, is sufiicient to destroy, in a great measure, the force 
of the argument in the former part of it : for, since Newton has 
demonstrated, that for rays of a given colour, the lengths of 
the fits in different media are proportional to the sines of refrac^ 
tion corresponding to a given angle of incidence out of a va- 
cuum, and since the more recent discovery of the different dis- 
persive powers of substances has proved that media differ very 
considerably in the proportion of these sines for the different 
rays of the spectrum, it follows that the proportional lengths of 
the fits must differ in every different medium. Hence will arise 
a difference in the scale of colours which thin laminse of diffe- 
rent media should exhibit ; and though we may certainly fix on 
one (that exhibited by a vacuum for instance) as a standard, and. 
call it the Newtonian Scale, yet this, though convenient, is ne- 
vertheless in some degree arbitrary, as we know not the nature 
of the media, with which what we call a vacuum may be filled, 
nor their action on light. Nor is this cause of deviation so small 
as to be safely neglected in all cases. In oil of cassia, the diffe- 
rence in the refractions of red and violet rays amounts to no less 
than of the whole refraction, and the colours exhibited by 
thin or thick plates of this liquid should therefore deviate very 
sensibly from those in air or in vacuo. Various solids, too, as 
chromate of lead, realgar, &c. could they be obtained by any 
means, in sufficiently thin leaves, ought to exhibit a scale of co- 
lour differing altogether from that of Newton. 
The very remarkable succession of colours exhibited by that 
variety of the Fish-eye stone (Apophyllite), which possesses a 
single axis of double refraction, has been noticed by Dr Brewster, 
and since shewn in my paper already alluded to, to indicate an 
action on polarized light, very nearly the same for all the co- 
lours, being equal upon the red and indigo-blue rays, a little 
greater for the yellow and green, and a little less for the violet,, 
being the only instance yet adduced in the whole circle of optw 
VOL. IV. NO. 8. APllIL 1821 . Y 
