and its Utility in. the Arts. 25 
rarely perfect, will form laminz, that may be eafily obferved 
on looking at the edge of the glafs. Undulating threads are 
almoft always found alfo at the joinings of the layers. Op- 
ticians diftinguifh two kinds of thefe threads, the firft of 
which they call ropes, becaufe they are full and of different 
denfities: they are always prejudicial to the goodnefs of op- 
tical infiruments. The fecond are lefs troublefome, but 
more common. They are capillary tubes, which produce 
two pencils of light in a direction perpendicular to their 
axes. 
_ Ihave already given, in a work entitled Memoires fur la 
Mecanique et la Phyfique, a method of obtaining large ob- 
je& glafles from thin plates of flint glafs, and of freeing 
them from threads when their difpofition in the fubftance 
of the glafs does mot render it impoffible. This procefs, 
which I have {till brought to greater perfection, confifts in 
expofing the elafs to a fire fufficiently ftrong to melt it, 
When the elafs is foft, it muft be rolled up with a pair of 
iron pincers, and formed into a cylinder three inches in 
thicknefs. Thofe folds which would infallibly refult from 
too rapid bending ought to be carefully avoided. The edges 
of the glafs mutt be kept in by a ring of clay. The cylinder 
muft then be left in the furnace a fufficient length of time to 
be annealed ; and when this operation is terminated, it mutt 
be polithed, in order to difcover its faults. It is then to be 
fawn through at the places moft filled with threads, and to 
be put into the furnace a fecond time to give it the proper 
form. 
The detail into which I have entered on the nature of 
glafs proper for optical purpofes, ought not to make us lofe 
fight of the great advantages which reflectors have over 
achromatic telefcopes. Since the difcoveries of Herfchel, 
the utility of the former cannot be doubted; but it is of 
great importance to employ, in the conftruétion of thefe in- 
ftraments, a metal the polifh of which may be bright and 
unchangeable, Twenty pounds of this precious metal, which 
were 
