34-2 . - QUt$l, &e. 



flintglass, as 36 to three. The mean refraction is also stronger, 

 being-Id, while that of flint glass is 16Q. 



We caused a piece of the new glass to be cut into lenses of 

 16() millimetres focus; and we can assure the Institute, that 

 this rigorous examination convinced us that France may now 

 dispense entirely with flint glass in the construction of good 

 achromatic glasses, so necessary in the naval and military 

 Service. The above glasses, which we have examined and 

 compared with English glasses, prove that the eulogies we 

 bestow on them are well founded ; we do not mean to say, 

 however, that, when taken hv.hscriminalely, the' glass made 

 byM. Doufourger-ais can be used for the large object-glasses 

 used by astronomers in delicate observations : in this case, 

 as in flint glass, it would be necessary to pick out a piece, in 

 order to avoid the air- drawn threads and streaks from which 

 blown glass is rarely free; and we ought not to require, in a 

 large mass of glass, -a perfection which is perhaps chimerical, 

 for instruments of no importance in a commercial point of 

 yiew, however valuable in scientific pursuits. We must ob- 

 serve, nevertheless,, that the glass made by M- Doufourgerais, 

 although very ponderous, has generally fewer threads than 

 flint glass, and its clearness equals, if if does not surpass, that 

 of English glass. 



. The largest glass made with the article manufactured by 

 M. Doufourgerais, and wnich we examined, was only eight 

 decimetres in length. Its object glass had an aperture of 60 

 millimetres, and it magnified the diameter of objects 30 

 limes. With astronomical eye-glasses we can make it ex* 

 hibit a much greater magnifying power: but this increase 

 is not of any use in the examination of terrestrial objects. 

 We doubt if it has all the aperture which it can support, 

 because opticians must change the ordinary proportions, 

 when they u.e a glass which has a stronger, dispersion than 

 $tras$. 



In 1 7 f 4 , Nicholas Fust published a work in French, with 



the following title : " Instruction detaillce pour porter les 



Lunettes de twites les differentes Especes au plus haut Dcgre 



jectlun dint flies dont susceptibles, tirte de la Theorie 



i l dioptrique 



