[ ' 9 ° ] 
teilite : for you know. Sir, that there is nothing to 
be depended on, in all that has been done upon the 
diameters -of the fatellites by Cafiini, Whifton, and 
Maraldi. The following is the way which I have taken, 
to determine thefe unknown quantities. I obferve, 
firfl of all, that two of them, a and b> are reducible 
to one ; becaufe, as you will fee prefently, the two 
fegments are always in a known proportion [to the 
whole difc of the fatellite, as well as to each other] ; 
and confequently, the proportion of their verled 
fines, Ab , AB, may be obtained, either by calculation, 
or by a table made for the purpofe. In order to 
difcover it, confidering that when the fatellite dis- 
appears, it is from the diminution of its light, I 
conceived, that one might contrive to imitate, at any 
time, what happens in the eclipfes, by diminifhing 
the light. I have an achromatic telefcope of five 
feet length, and 24 lines aperture. I made fome 
diaphragms of pafteboard, which I could apply on 
the 
Monfieur, tout ce qui a ete fait fur les diametres des fatellites 
par Caffini, Whifton, et Mr. Maraldi, et vous favez qu’il n’y 
a rien fur quoi on puifle compter. Voici comment je fuis par- 
venu a les determiner. Je remarque d’abord que les deux in- 
connues, a, b, fe reduifent a une feule, parceque, comme vous 
le verrez tout a l’heure, les deux fegmens font toujours dans un 
rapport connu, et confequemment par le calcul ou par une 
table dreflee expres, on a le rapport de leurs ftnus verfes Ab> 
A B. ]Pour les connaitre, j’ai vu que quand le fatellite difparait, 
c’eft: par la diminution de fa lumiere, et j’ai penfe, qu’a tous les 
momens on pouvait imiter, d’une maniere fadfice, ce qui ar- 
rive dans les eclipfes, en diminuant la lumiere du fatellite. 
J’ai une lunette achromatique de 5 pieds, qui porte 24 lignes 
d’ouvert'tre. J’ai taille des diaphragmes de carton, que je pouvais 
appliquer 
