Dr. Fitton's Notes on the History of English Geology. 14-7 



tions make the apparent inclination in Arragonite a little more 

 than 32°, whereas the theory makes it fully 34;°. According 

 to my observations, the apparent inclination was 31° 15', and 

 the real inclination 18° 18', computed with an index of 1'693. 

 Upon re-examining this crystallized plate some years after- 

 wards, I found that its surface was inclined to the axis A ; and 

 upon measuring the inclination, I found that the true inclina- 

 tion of the optic axes was 17° 33', which corresponds to an 

 apparent inclination of about 29° 56' for the mean ray of the 

 spectrum. Different crystals, however, have different inclina- 

 tions; so that we are not entitled to compare this result with 

 that which is deduced from theory. 



The discrepancy between theory and observation is still 

 greater in topaz, amounting to 8° in the inclination of the 

 optic axes, if the specimen used by M. Rudberg had the same 

 structure as that which was used by M. Biot and myself. 

 Taking the index of topaz at 1*636, 1 found the apparent in- 

 clination of the axes to be 121° 16', and the real inclination 

 64'° 22', which, from the excellence of the specimen which I 

 used, cannot, I think, err above half a degree. M. Rudberg 

 will no doubt measure the inclination of the axes in every 

 specimen by which he has obtained the theoretical inclination. 



In expressing a hope that M. Rudberg may continue his 

 valuable observations, with other crystallized bodies, we trust 

 he will excuse us for adding, that though the subject of double 

 refraction is under the deepest obligation to M. Fresnel, yet 

 other philosophers have wrought in the same field before him ; 

 and that his transcendent merits would not be diminished by 

 a proper recognition and acknowledgement of their antecedent 

 labours. 



XXXIII. Notes on the History of English Geology. By 

 William Henry Fitton, M.D. F.R.S. S,-c. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, c-, 



A N article which I published in the Edinburgh Review for 



-^^ February 1818, On the Geological Map, and other works 



of Mr. William Smith*, having been frequently referred to, 



the historical part of it was printed, with some additions, in 



t The following is a list of Mr. Smith's works, prefixed to the article in 

 the Review, — vol. xxix. |). .'JIO, &c. 



'• 1. A Delineation (Map) of the Strata of England and Wales, wfttj 

 Part of .Scotland; exhibiting the ('oliicries and Mines, the Marshes an<<" 



Fcnlandioriyinall}' overflowed bv the 8ca, and the Varieties ofSoil aitord- 



U 2 



