48 Mr. J. F. W. Herschel on the action of 
severe as the most splendid specimen, and thus extending 
our researches to an infinitely greater variety of natural 
bodies, than we could otherwise hope to examine. 
In order, however, to render observations on the tints de- 
veloped by polarised light available, they must be comparable 
to each other ; and it therefore becomes an object of the first 
importance, to ascertain the existence, and discover the laws 
of any causes which may operate to disturb their regularity. 
Ever since I first engaged in experimental enquiries on the 
polarisation of light, I was struck by the very considerable 
deviation from the succession of colours in thin laminse, as 
observed by Newton, which many crystals exhibit when cut 
into plates perpendicular to one of their axes. I at first 
attributed this to a want of perfect regularity in their struc- 
ture, or to inequalities in their thickness, arising from my 
own inexpertness in grinding and polishing their surfaces ; 
and it was not till habit had rendered me familiar with all the 
usual causes of deception, that, finding the same phenomena 
uniformly repeated in different and perfect specimens, my 
curiosity became excited to enquire into their cause, the more 
so, as they now began to assume the form of a radical and 
unanswerable objection to the theory of M. Biot, above al- 
luded to, which affords so perfect an explanation of the tints 
in crystals with one axis. 
These phenomena have not escaped the vigilance of Dr. 
Brewster. In his paper of 1818, he distinctly notices the fact 
of a deviation from Newton's scale, in crj'stals with two axes, 
and promises a more detailed account of it, which however has 
not yet appeared. But the object of the present communication 
is not thereby anticipated, as in the only passage in that paper 
