212 Le Comte de Bournon oh [Sept. 



to inflate the silk was seven or eight times lighter than common 

 air, which would of course make a great difference ; though as 

 the second objection of the machine collapsing on approaching 

 the earth could not be got rid of, I am apt to believe him wrong- 

 in his calculations. 



Article X. 



Description of the improved Goniometer of M. Adelmann.* 

 (With a Plate.) 



Thf. first goniometer was that of Carangeot, which was made 

 under the direction of Rome de Lille, who may justly be regarded 

 as the father of crystallography. This instrument (the only one 

 ever used by Haiiy) was much improved by M. Gillet de Lau- 

 mont, who increased the size of the semicircle, and made the 

 divisions more sensible, but still it partakes of the imperfections 

 of the original instrument. One of these consists in the diffi- 

 culty of placing the two legs of the instrument perfectly perpen- 

 dicular to the edge which unites the two planes of a crystal, 

 whose angle of incidence we wish to measure ; another in plac- 

 ing them on the faces of the crystal, so that the whole flat sur- 

 face of the legs, and not merely their edges, shall touch the face 

 of the crystal in every point, without which it is impossible to 

 obtain an accurate measurement. 



" The reflective goniometer, for which we are indebted to 

 Dr. Wollaston, to whom the sciences owe so many other obliga- 

 tions, is a much more perfect instrument, but it requires condi- 

 tions in the crystals to be measured by it, not easily met with ; 

 such as perfectly plane faces, free from strife, and sufficiently 

 brilliant to reflect the light in such a manner as to present a 

 distinct image of the lines of observation. If we attempt to 

 measure a crystal with this instrument, whose faces are striated, 

 however slightly, which unfortunately is but too commonly the 

 case, or which are not perfectly flat, we may be certain of 

 obtaining results more or less inaccurate — a circumstance which 

 it seems to me has already often occurred.f I admit, however, 



* Extracted from the Memoir of M. Le Comte de Bournon. 



■f The author of the Review of the third edition of Phillips's Mineralogy (Journal of 

 Science, vol. xv. p. 324) seems to entertain very different notions of the merits of the 

 reflective goniometer from those of Count Bournon. The review contains a copy of Mr. 

 Phillips's elaborate figure of a crystal of humite, selected, says the author, " first, 

 because its form has never been described before;" secondly, Count Bournon, in his 

 Catalogue, says, that " all its planes arc striated, whereas not. one of them is so ; for 

 what he mistook far striae, arc, in fact, so many planes, as has been proved by subjecting 

 the crystals to the reflective goniometer ; thirdly, it shows, therefore, the value of that 

 instrument in a striking degree, and that the use of it quickens the sight of the observer, 

 who, while measuring without a glass, finds planes, where an old, and generally sup- 

 posed accurate observer saw only striae."— J. G. C. 



