1821.] Crystallized Bodies on Homogeneous Light. I IT 



deceptive discussion of tabulated measures, but on the actual 

 form of the curves themselves, which are loci of the functions, 

 under consideration. It is true, that a reference to tabulated 

 measures is indispensable to give precision to such first approxi- 

 mations ; but the powder this mode of observation affords of 

 copying our outline fresh from nature, and from the general 

 impression of the phenomena, brought at once under our view, 

 is an advantage not to be despised. Nor ought we, lastly, to 

 omit, in our estimate of advantages, the means thus afforded us 

 of subjecting the minutest fragments of a crystal to a scrutiny 

 as 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 deve- 

 loped 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 inquiries on the polarisa- 

 tion of light, I was struck by the very considerable deviation 

 from the succession of colours in thin laminae, as observed by 

 Newton, which many crystals exhibit when cut into plates per- 

 pendicular to one of their axes. I at first attributed this to a 

 want of perfect regularity in their structure, or to inequalities in 

 their thickness, arising from my own inexpertness in grinding 

 and polishing their surfaces ; and it was not till habit had ren- 

 dered 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 inquire 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 alluded 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 crystals with two axes, and 

 promises a more detailed account of it, which, however, has not 

 yet appeared. But the object of the present communication i» 

 not thereby anticipated, as in the only passage in that paper in 

 which he expresses himself otherwise than obscurely on its 

 cause, he appears to regard the deviated tints as analogous to 

 those developed along the axis of rock crystal and by certain 

 liquids ; an analogy which, in the present state of our knowledge 

 on that perplexing subject, it seems not easy to admit. In a 

 paper too, which has lately appeared, containing the interesting^ 

 observations of the same excellent philosopher on the optical 

 structure of the apophyllite, he remarks the very striking devia- 

 tion of the colours of this crystal from Newton's scale ** in the 



