[ 21 S ] 
when it is in the {mailed: degree diminiffied, i. e. when the very 
fmallcft part imaginable of the dife is fhaded. Therefore, the 
moment of the cifpariticn, in the aperture of the diaphragm, is 
the true commencement of the eclipfe, or differs from it by lefs 
than any affignable difference.” 
But it appears, from the examples given afterwards, that the 
author’s calculations proceed upon much faff r principles. Having 
determined the portion of the difc, that is infenffble on the whole 
given aperture of his telefcope, he computes what larger portion 
will be infenffble, on the fmaller given aperture of his diaphragm. 
And then, by obferving the two difparitions, the earlier one in 
the diaphragm, the other in the telefcope with the object-glafs 
uncovered', the laft of which he calls the true immeifion, he 
knows the time, in which a given portion of the diameter enters 
the fhadow, and, confequemiy, the time, in which the whole 
enters; whieh determines the magnitude of the whole, in parts 
of the fatellite’s orbit, or its apparent magnitude to an obferver 
at Jupiter’s center. 
( c ) The difadvaritage of ufing too great 2n aperture is, 
that the part of the diameter obtained by cbfervation, from 
which the whole is to be concluded, will be lefs, than the fame 
method of obfervation would give, with a more contracted 
aperture. For the larger the aperture of the diaphragm is, 
which is applied to the object- glafs, the lefs is the difference 
between that aperture and the whole aperture of the tele- 
fcope ; confequently, the lefs is the difference between the feg- 
ments, which are infenffble in thefe apertures feverally, and the 
lefs the portion of the diameter, which pafies over the fhadow’s 
edge, between the two difparitions. 
[ d ) In eclipfes, when once the fatellite hath difappeared, or is 
become viffble, the author fays, we are not to e.xpedt t'nofe fits of 
glimmering and extinction, which he hath defciibed as taking 
place, when we obferve the uneclipfed fatellite with very con- 
tracted apertures. 
The reafon is plainly this. In immerffons, a part of the difc 
is ffill indeed enlightened, when the fatellite difappears; and the 
quantity of light, tranfmitted from this part to the obferver’s 
eye, muff be very different, in different ftates of the air’s trans- 
parency'; and confequently, the fatellite, after having difap- 
peared, might become viable again, by a fudden increafe of the 
air’s tranfparency in the tract of the fittelike’s light, provided, 
the 
