C 3 6 + 3 
pearing to the naked eye, as an arc of a greater circle, 
than the other, which receives the reflex light from 
the earth. Look thro' a refracting telefcope, and you 
will perceive the apparent difference of thefe circles 
very much diminilhed : and if they be view’d with 
a good refleCtor, they will be perfectly reduced to an 
equality, even if meafured with a micrometer in the 
focus, as we have often found. 
If a reflecting telefcope, well conftruCted in all re- 
fpeCts, be directed any conflderable time to the fun, 
fuch a circle of aberration will be generated, from the 
little fpeculum’s being heated, and thereby its figure 
alter’d, from the fun’s rays falling condenfed thereon 
from the great one ; and if it continues long under 
this circumflance, the image will be render’d utterly 
indiftinCt and confufed. 
This we were thoroughly convinced of at the 
above-mention’d tranfit of Mercury : for the refleCtor, 
a very good one, which we ufed, in taking, with the 
the micrometer, the differences of right afcenfion and 
declination between the planet and the fun’s limb, 
having been a good while expofed to the direCt rays, 
was found at laft to give a very indiftinCt image ; 
but was reftored to its former degree of perfection, 
by turning it from the fun, and fcrewing off the eye- 
piece, fo as to admit the cool air into the great tube, 
whereby the over- heated fmall fpeculum foon reco- 
vered its due temper and figure. The laft- mentioned 
effect is fcarce fen Able in the leffer reflectors of fmall 
apertures 3 but in thofe of large ones it is very confi- 
derable. 
Dr. Bevis, Mr. Canton, and Mr. Bird, who viewed 
Mercury going off the fun, with very good reflectors 
ot different lengths, affure me, they law him quite 
diftinct 
