and its Utility in the Arts. 195 
labour and: much money, Iwas not able to continue this 
kind of obfervations. | 
This telelcope of feven feet, which I confider as fuperior 
to that of Herfchel of the fame length, enabled me, during 
the time I had the ufe of it, to be convinced that the {pet 
Ariftarchus, if not a volcano, is at leaft a body luminous of 
itfelf. The grounds on which I found my opinion are agree- 
able to the principles explained by Bouguer in his Treatife 
on the Gradation of Light; for thofe {pots of the moon 
which reflect moft ftrongly the dire& light of the fun did 
not exhibit to mein the hade the fame appearances as Arif- 
tarchus, whatever efforts I made to augment the intenfity 
of that feeble light which arifes from the reflected rays of 
the earth. Befides, it is eafy to conceive that a very lively 
fire, concentrated in.the hollow of a crater, cannot diffe- 
minate the light which emanates from it: in that parti- 
cular cafe, like a reverberator, it cannot be feen but under 
certain circumftances. It is remarked then, that it is in 
the months of Pluviofe, Ventofe and Germinal that this 
volcano is moft apparent. 
When f undertock a journey to London, by order of Go- 
vernment, for the benefit of the feiences, I employed my- 
felf ina particular manner in the improvement of flint glafs; 
and I, was convinced that the propereft and fimpleft means 
“for rendering‘flint glafs fit for conftructing large achronvatic 
telefcopes confift in removing the threads by means of-a 
glafs-cutter’s wheel. When thefe faults are removed, the 
glafs is to be kneaded in an oven and under a muffle, in 
fuch a manner as to give it almoft the form and fize of the 
“obje&-clafs’ required to be made. This procefs is amply 
deferibed im a work which I publifhed in 1783, entitled 
Recueil de Memoires fur la Mechanique ct la Phyfique. 
It was in this collection that I faid that Euler had been 
the firt who thought of correcting the aberrations of re- 
frangibility by ufing fubftances of different refractive pow- 
ers. Maupertuis engaged to get conftructed at Paris Eu- 
ler’s 
