Some Account of the late M. Guinand. 25'i 



a hand-barrow to a small water saw-mill, which he had esta- 

 blished at the fall of the Doubs, at the distance of half a league 

 from his abode, the mass slipped from its bearers, and rolling to 

 the bottom of a steep and rocky declivity, was broken into seve- 

 ral pieces ; M. Guinand was at first grieved at this mishap, but, 

 having selected those fragments which appeared to be perfectly 

 homogeneous, he succeeded in softening them in circular moulds 

 in such a manner that, on cooling, he obtained disks that were 

 afterwards fit for working. Thenceforward he adhered to this 

 method ; and he also contrived a mode of cleaving his glass while 

 cooling, so that the fractures should follow the most faulty parts. 

 At his house are frequently to be seen pieces of unwrought glass, 

 weighing from forty to fifty pounds, perfectly transparent and 

 homogeneous. When flaws occur in these, he removes them by 

 cleaving the pieces with wedges ; hcf then melts them again in 

 moulds which give them the form of disks, generally taking care 

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

 of the edge, so that the opticians who work them may be enabled 

 to use that portion of glass in making a prism, which shall give 

 them the measure of the refractive power, and thus obviate the 

 necessity of cutting the lens. For 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. 

 This mode of fabricating glass from pieces rough and shapeless 

 in the first instance, and afterwards re-molten into disks, renders 

 the process of M. Guinand absolutely different from that of other 

 glass-founders, who proceed either by casting or blowing. 



When the disks obtained by this process are still defective, as 

 it sometimes happens, especially with those of great dimensions, 

 M. Guinand obviates the defects by means of the wheel (roulette) ; 

 then by softening them again the vitreous matter expands, and 

 fills up the hollows that have been made ; if after polishing he 

 finds them still defective, he repeats the process until the disks 

 are as perfect as he could wish. By these means he has often 



