a4 Obfervations on Platina, 
only flint and common glafs in the conftruction’ of achro-' 
matic telefcopes, it is certain that the fame object might 
be attained with other tranfparent fubftances, both folid and 
fluid*; but, as the queftion propofed by the Academy has in 
view only the improvement of flint glafs, I fhall here offer 
fome obfervations, which may be of ufe to thofe who with 
to become candidates for the prize t. 
Flint glafs is brought to us from England in very thin 
plates. It is blown into globes, which are cut up and 
ftretched out when they have attained tothe proper fize and 
thicknefs. Blown glafs will confift of parallel layers, if the 
workman is not able to take up at once the neceflary quan- 
tity of matter; and thefe layers, the junction of which is 
* This idea has been realifed by Dr. Blair of Edinburgh. Flint glafs 
refléts the green light confiderably lefs than crown glafs, in proportion to 
the whole refraction of red and violet light; fo that when the divergency 
of the red and violet light caufed by the refra€tion of the two mediums is 
equal, the divergency of the red and green light is always greater in the 
crown than in the flint glafs, and the divergency of the violet and green 
light is always lefs in the crown than in the flint glafs, After a variety of 
experiments, Dr. Blair difcovered that the muriati¢ and nitric acids, which 
are difperfive Huids of confiderable ftrength, inftead of refracting the green 
dight lefs than crown glafs, in proportion to the whole refra€tion of the red 
and violet light, refraéted the green light more than crown glafs in pro- 
portion to the whole refraction of red and violet light: he therefore mixed 
thefe two kinds of difperfive mediums, and thus obtained a medium which 
difperfes the rays much more than crown gla{s, and yet caufes them all te 
diverge accurately jn the fame proportion in which they are made to diy 
verge by the refraction of crown giafs; which entirely removes the aberray 
tion from the unequal refrangibility of light. For this difcovery Dr. 
Blair has obtained a patent. Eprr. 
+ This memoir, the author tells us ina note, was read on his public ade 
riffion into the Academy of Sciences at Paris, in the year 1786. It was 
printed in the fEcond part of his Voyage to Madagajcar, but that part, 
owing to an aceident which could neither be forefeen nor prevented, was 
never given tothe public. It contained nautical details, obfervations on 
the charts of India, and a differtation on the AT HMEnTS which may 
be ufeful or neceflary for the navigator, 
rarely 
