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account for them all, from this fingle fuppofition > 
namely , that the difks of the Sun, and of Mercury 
leen thereon, are environed with a certain corona of 
light (like that which Sir Ifaac Newton calls the cir- 
cle of aberration or diffipation in refra&ing telefcopes) 
whereby the apparent diameter of the Sun is inlarged,’ 
and that of Mercury contracted. 
But, as this gentleman made ufe of a reflecting 
teleicope, and as no fuch circle, from the known 
principle of reflexion, can take place in fuch a tele- 
icope, if well made, as Sir Ifaac has proved long 
ago ; I fhall not take up the Society’s time in purfu- 
mg him thro’ all his particular fuppofitions ; but lhall 
endeavour to fhew, that his hypothefis has really no 
foundation. J 
Sir Ifaac, as I before hinted, in feveral of his writ- 
ings, remarks, that the images of all objeCts feen in 
refracting telefcopes are furrounded with a circle of 
aberration ; which is always lefs, the longer the tele- 
fcopes are. In his optics he tells us, that, to avoid 
the indiflinCtnefs arifing from this circle, he would 
propofe catadioptric telefcopes, in which, if the fpe- 
culums, under limited apertures, bejufily figured, no 
fuch circle of aberration can confufe the imaoe : but, 
if the fpeculums are of a fpherical figure, with too 
large apertures, then indeed a circle of aberration 
will take place j as it alfo will, when the figure de- 
viates fiom the circular towards the hyperbolic, even 
under a fmall aperture ; and the fame thing will hap- 
pen, if the fpherical figure be inaccurate. 
About three days from the change of the Moon, 
her whole body is vifible : that part of the limb, 
which is direClly enlightened by the folar rays ap- 
Z z 2 pearing 
