[ *97 ] 
fegments, muft be computed, and in the expreffion 
2 R r (a — b) > t j ie on jy rerna i n j n g unknown quantity, 
will be r. The following is the method I have hit 
upon for determining it. 1 confidered, that, by trying 
different diaphragms fucceffively, fome few minutes 
before an immerfion, it would be eafy to find out 
the particular fize, which would make the fatellite 
difappear ; and that the proportion of the invifible 
fegment to the whole difc of the fatellite, for that 
inftant, would, by that means, be determined. Sup- 
pofe then that I have found this diaphragm : my 
next ftep is, to cover the objeft-glafs of my tele- 
fcope with a diaphragm fomewhat larger, which 
fuffers me juft to perceive the fatellite, but fo weak 
and fmall, that the leaft further diminution of its light 
muft render it invifible. I wait till it actual- 
ly difappears j I write down this time, then take 
away the diaphragm, and the number of feconds, 
which pafs betwixt this firfl difappearance and 
the 
2Rr (a — b ) 
on n’aura plus d’autre inconnue que r. Void 
la methode que j’ai imaginee pour la determiner. J’ai penfe, 
que fi quelques minutes avant une immerfion, on eflViait dilv- 
ferens diaphragmes, jufqu’ace qu’on ait trouve celui, qui fait dif- 
paraitre le fatellite, on aurait, pour ce moment, le rapport du feg- 
ment invifible au difque entier du fatellite. Ce diaphragme 
etant connu, je couvre enfuite l’objedtif de ma lunette d’un dia- 
phragme un peu plus grand, qui me laille appercevoir le fatellite, 
mais faible et tres petit, de maniere que ce fatellite ceffera d’etre 
vifible des que fa lumiere fera tant foit peu diminuee. J’attends 
qu’il difparailfe, [et je fuis ainfi avert! du moment ou il commence i 
toucher I’ombre ] [b) je marque cet inftant, j’enleve !e diaphragme, 
et le nombre de fecondes ecoule, entre cette premiere difparition 
D d 2 et 
