a new Micrometer and Megameter. 791 
meeting of the Academy. This the Abbe fontana im- 
mediately told me, and I repeated what 1 had laid to him 
the firft time on the effedt of this micrometer; adding 
at the fame time the precife meafure of the fcale for the 
meafure of the fame effect, and the facility of obtaining 
the fame thing without the rock cryftal; mentioning 
likewife other advantages which might be derived from 
the common glafs prifm not covering the whole aper- 
ture of the objedt-glafs ; and, amongft others, that of 
being able to meafure much greater angles by this means 
than by the double refradfion of the rock cryftal. 
The Abbe rochon did accordingly read his paper to 
the Academy, and mention has been made of it in the 
public prints : he has therefore the merit of having thought 
of the fame thing, at the fame time with, or perhaps 
before 
fur cet obiet, pour lire a la prochaine feance de l’Academie. Mr. I’ Abbe fon- 
tana eut la bonte de m’en avertir immediatement; ce fut pour lors.que je lui 
repetal ee que j’avois eu 1’honneur de lui dire la premiere fois fur I’efFet de ce 
micrometre, en y ajoutant la mefure precife de 1’echelle.pour la mefure du meme. 
effet, et la faciiite d’obtenir la meme chofe fans le criiiai de roche, avec d’autres 
ayantages que l’on pouvoit tirer du prifme a verre fimple, ne couvrant pas toute 
1’ouverture de Tobje^lif, et entre autres celut de pouvoir mefurer par ce moyen 
des angles beaucoup plus grands, que par la double refraction du criitai de 
roche. 
M. l’Abbe rochon a reellement lu a TAcademie fon memoire, et on en a fait 
mention dans les gazettes: ainii il a le merite d’ avoir imagine la meme chofe. 
