[ ^ ] 
the true immerfion, giving me a great part of 
the diameter, I eafily compute the whole. The 
following is an example of my method. On the 
26th of June 1771* there was an immerfion of the 
third fatellite, at 56' after nine in the evening. I 
found the diaphragm, which made the fatellite dis- 
appear, to be of 12 lines. Then I fitted my glafs 
with a diaphragm of 17 lines ; I might have taken 
one much fmaller : presently the fatellite difappears. 
But removing the diaphragm, I fee the fatellite again, 
very diftindtly, for 2' 18"; after which, the true im- 
meriion followed. Now, this is my calculation. The 
aperture of the diaphragm, which made the fatellite 
difappear, being 1 2 lines by obfervation, the invifible 
fegment at the inftant of the eclipfe, mull: have been 
a quarter of the difc. Let 
A B D be this quarter. I 
know that, at the inftant 
of the immerfion, the fa^ 
tellite had entered the fhade, 
by the whole part E F of 
its diameter. I fay then, if 
on an aperture of 24 lines, 
the part A B D is infenfible, 
the infenfible part, on an 
aper- 
et la veritable immerfion, me donne la mefure d’une grande par- 
tie du diametre, d’ou il eft aife de conclufe le diametre entier. 
Void un exemple de la methode. Le 26 Juin, 1771, a 9 h. 
56' du foir, il y avait une immerfion du troifieme fatellite. J’ai 
determine de r2 lignes le diaphragme qui faifait difparaitre le 
fatellite. J’ai garni ma lunette d’un diaphragme de 17 lignes 
(je l’aurais pu prendre beaucoup moins ouvert) j’ai vu difparai- 
tre le fatellite, j’ai ote le diaphragme, j’ai revft, tres diftin&ement, 
le fatellite pendant 2' 18", apres quoi s’eft faite la veritable im- 
merfion. Void le calcul, J’apprends, par l’obfervation du dia- 
phragme 
