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Somewhat analogous to this is this other appear- 
ance : If you look thro’ a telefcope at any of the 
planets, when the ftars appear hazy, dim, and lan- 
guid, you will fee them diftindly : but look at them 
again, when the ftars appear mofl bright and fpark- 
ling, you will then find their images lefs diftind. 
This may be accounted for by the juft-mention’d 
inlfance of the veiTel of water, by fuppofing air in- 
stead of water. And if we confider the infinite num- 
ber of heterogeneous particles which continually float 
in the air, and fuppofe thefe to be at reft, or put 
into motion, we lhall find, that it is not at all 
fiurprifing, that we fee the images of objects, placed 
beyond the medium of air, more or lefs diftind. 
We are not l'o fenfible of this indiftindnefs, arifing 
from the agitation of the air, in refrading telefcopes, 
as in refledors : becaufe the errors of reflexion, caufed 
by any irregularity in their figure, or confufion in the 
air, are about five or lix times greater than the fame 
errors in refraction ; even tho’ both telefcopes magni- 
fy the fame number of times ; as has long ago been 
demonftrated. 
We alfo took notice of M. de Barros’s firft pheno- 
menon ; to wit, the feeming greater velocity of Mer- 
cury when he was near the egrefs : which we thus ac- 
counted for. When he was at a confiderable diftance 
from the limb, there being nothing near enough to 
refer his velocity to, he feemed in a manner fla- 
tionary ; but being advanced near the fun's edge, we 
could refer his motion to that, with eafej which 
thus becoming fenfible, it might be efteemed rapid, 
in companion of the former. I have often made the 
iame remark on the gradual approach of two lumi- 
nous 
