792 -Abbe boscowich’s Account of 
befoi'e me, without any knowledge whatever of my ideas 
on the fubjeCt; he has been the firlf who announced it to 
the world, who had it executed, and who made ufe of it : 
I have thei'efore no pretenfions whatfoever on that head ; 
he has the merit of a great dilcovery, and aftronomy has 
the foie obligation of it to him. 
But the Abbe rochon has only made ufe of the dou- 
ble refraction of the rock cryltal for his micrometer, and 
I am allured he has faid, that his prifm could give him no 
more than fix degrees. Now it is well known, that pieces 
of rock cryltal, large enough and pure enough for thefe 
purpofes, are extremely rai'e; befides, the difficulty of 
working them is great, that fubltance being harder than 
glafs, and requiring the utmolt attention in cutting, in 
order to obtain the difference defired between the two 
refractions. I think therefore, that it will be doing an 
effential 
dans le meme terns, pent etre avant moi, et abfolument fans avoir en aucunecon- 
noifiance de mes idees fur le meme objet ; de 1’ avoir annonce le premier au public, 
de 1’avoir execute, et de s’en etre fervi le premier: ainfi je n’ai rien a pretendre 
de ce cote la: il a le merite d’une belle decouverte, et l’afironomie lui en a 
toute l’obligation. 
Mais M. l’Abbe rochon n’a employe, pour fon micrometre, que la double 
refraction du criftal de roche; et on m’a afiure qu'il a dit, que fon prifme ne 
pouvoit lui donner que jufqu’a fix degres. On fait bien que les pieces afiez 
orandes de cette matiere, et afiez pures, font tres rares; outre la difficult^ de la 
travailler, etant plus dure que le verre, et quelle attention il faut avoir pour la 
bien couper afin d’avpir la difference des deux refraCtions que Ton veut. Ainfi 
