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equalities, which will naturally arife from the chance 
of the rays coming fometimes a little denfer and 
fometimes a little rarer, in fo fmall a number of 
them as mull fall upon the eye in the fourth or fifth 
part of a fecond, may be fufficient to account for this 
appearance ? An addition of two or three particles of 
light, or perhaps of a fingle one upon twenty, efpe- 
cially if there fhould bean equal deficiency out of the 
next twenty, would I fuppofe be very fenfible : this 
feems at lead; probable from the very great difference 
in the appearance of ftars, whofe light is much lefs 
different than, I imagine, people are in general aware 
of; the light of the middle-moff ftar in the tail of the 
great Bear does not, I think, exceed the light of the 
very fmall ffar next to it, in a greater proportion than 
that of about fixteen or twenty to one ; and Moniieur 
Bouger tells us, in his Traite d’Optique before-men- 
tioned, that he finds a difference in the light of objedts 
of one part in fixty-fix fufficiently diffinguiffiable. 
It will perhaps be objected, that the rays coming 
from Sirius are too numerous to admit of a fufficient 
inequality, arifingfrom the common effedt of chance, 
fo frequently as would be neceffary to produce this 
effedt, whatever might happen in refpedt to the fmaller 
ftars j but till we know what inequality is neceffary to 
produce this effedt, we can only guefs at it either one 
way or the other ; there is however another circum- 
ffance, that feems to concur in the twinkling of the 
ftars, befides their brightnefs, and this is a change of 
colour. Now the red and blue rays being very much 
fewer, I apprehend, than thofe of the intermediate 
colours, and therefore much more liable to inequality 
from the common effedl of chance, may help very 
much to account for this phamomeribn, a fmall excefs 
Vol. LVII. L 1 4 or 
