79 ° Abbe boscowich’s Account of 
made variable. I added that the fame effect might be 
produced without the double refraction of the rock 
cryftal, with a prifm made of common glafs, only fmal- 
ler than the aperture of the objedt-glafs. The rays 
which pafs through the prifm would then form an image 
which would be feen out of its natural place; and thofe 
which pafs without would give another image, in the 
fame place it would have appeared in, if the prifm had 
not been there. 
A few days after, the Abbe fontana was told, that 
the Abbe rochon had thought of another micrometer 
with a prifm of rock cryftal, which, approaching more 
or lefs to the focus of the objeft-glafs, had the advan- 
tage of producing a very great effeft, and might be 
achromatic; that having accordingly had his inftrument 
executed, and made leveral obfervations with it, he had 
prepared a paper on the fubjeft, to be read at the next 
meeting 
ajoute que Ton pouroit avoir le memc effet fans la double refraftion du criftal 
de roche, en faifant un prifme a verre fimple, mais plus petit que Touverture 
de Fobje&if. Les rayons qui paffent par le prifme, formeroient alors une image 
deplacee defapolition naturelle, et les autres q\*i pafferoient dehors, donneroient 
Fautre image, ala meme place qu’elle auroit, s’il n’y avoit point de prifme. 
Quelques jours apres on a annonce a M. fontana que M. l’Abbe rochont 
avoit imagine un autre micrometre a prifme de cridal de rocne, qui, s’ap- 
prochant plus ou moins du foyer de Fobjeftif, avoit Favantage de produire un 
effet tres grand, et de pouvoir etre acromatique: qu’ayant fait executer fon 
inftrument, et fait avec lui plufieurs obfervations, il avoit prepare un memoire 
fur 
