352 Analysis of Scientific Books and Memoirs. 



instance even of eighteen inches in diameter, and having no douht that, 

 in operating on a greater scale, he might easily be able to obtain one of a 

 diameter double or triple the extent of those last mentioned, he concludes 

 that his process has at length removed the obstacle which the non-homo- 

 geneity of flint-glass opposed to the construction of large achromatic ob- 

 ject-glasses. 



When M. Guinand first obtained blocks including portions of good 

 glass, his practice was to separate them, by sawing the blocks into sections 

 that were horizontal, or perpendicular to their axis ; then polishing the 

 sections he selected the portions adapted to his purpose, and returned the 

 others to the crucible ; but, independently of its tediousness, and the waste 

 occasioned by sawing, this process was attended with the disadvantage of 

 not cutting the finest parts of his glass in the manner best calculated for 

 large disks ; for frequently the most homogeneous parts were thus divided. 

 A fortunate accident, however, of which he availed himself, conducted him 

 to a better process. 



While his men were one day carrying a block of this glass on a hand- 

 barrow to a saw-mill which he had established at the fall of the Doubs, 

 at the distance of half a league from his house; the mass slipped from its bear- 

 ers, and, rolling to the bottom of a steep and rocky declivity, was broken to 

 pieces. M. Guinand was at first grieved at this misfortune, but having se- 

 lected those fragments which appeared to be perfectly homogeneous, he 

 softened them in circular moulds in such a manner, that on cooling he ob- 

 tained disks that were afterwards fit for working. To this method he ad- 

 hered ; and he contrived a way of cleaving his glass while cooling, so 

 that the fractures should follow the most faulty parts. When flaws occur 

 in the large masses, he removes them by cleaving the pieces with wedges, 

 he then melts them again in moulds, which give them the form of disks, 

 taking care to allow a little of the glass to project beyond one of the points 

 of the edge, so that the optician may be enabled to use that portion of glass 

 in making a prism, which shall give him the measure of the index of refrac- 

 tion, and thus obviate the necessity of cutting the lens. The refraction of 

 M. Guinand's glass varies almost at every casting, while, on the other 

 hand, that of each casting is of such homogeneity, that the refractive force 

 of two pieces taken indifferently, one from the top and the other from the 

 bottom of the crucible, is absolutely the same. 



M. Guinand removes the defects by means of the wheel ; then by re- 

 softening the disks, the vitreous matter expands and fills up the hollows that 

 have been made ; if, after polishing, he finds them still defective, he re- 

 peats the process until the disks are perfect. By these means he has often 

 succeeded in soldering pieces of glass which have left no trace of their se- 

 paration : at first these pieces were only cemented ; there was frequent- 

 ly even air or sand between the united surfaces ; in these cases, he cut 

 along the line of junction a small semi-cylindrical groove, in order that the 

 vitreous matter, while melting, might fill it, not by flowing from its edges 

 to the bottom, but by raising the bottom itself, and by repeating this ope- 

 ration he declares that he has succeeded in totally effacing all traces of 

 junction. 



