L 262 3 
fied % I (hall offer the following, which may not per- 
haps be found an inadequate caufe of that appearance ; 
at leaft it has undoubtedly fome ffiare in producing 
it, efpeciaiiy in the fmaller ftars. 
It is not, I think, unreafonable to fuppofe, that a 
fingle particle of light is fufficient to make a fenfible 
impreffion upon the organs of fight. Upon this 
fuppofition, a very few particles of light, arriving at 
the eye in a fecond of time, will be fufficient to make 
an objedt vifible, perhaps not more than three or four; 
for though the impreffion may be confidered as mo- 
mentary, yet the perception, occafioned by it, is of 
a much longer duration : this fufficiently appears 
from the well-known experiment of a lighted body 
whirled round in a circle, which needs not make many 
revolutions in a fecond, to appear as one continued 
ring of fire. Hence then it is not improbable, that 
the number of the particles of light, which enter the 
eye in a fecond of time even from Sirius himfelf, may 
not exceed three or four thouland ; and from liars of 
the fecond magnitude, they may therefore probably not 
much exceed an hundred. Now the apparent increafc 
and diminution of the light, which we obferve in the 
twinkling of the ftars, feems to be repeated at not very 
unequal intervals, perhaps about four or five times in a 
fecond : why may we not then fuppofe, that the in- 
* Some aftronomers have lately adopted, as a folution of this 
appearance, the extreme minutenefs of the apparent diameters 
of the fixed ftars, which, they fuppofe, muft, in confequenee ot 
this, he intercepted by every little mote, that floats in the air ; 
but, that an object fliould be able to intercept a ftar from us, it 
niuft be large enough to exceed the apparent diameter of the 
ftar by the diameter of the pupil of the eye; fo that, if the 
ftar was a mathematical point, it muft ftill be equal in fize to the 
pupil of the eye. 
equalities. 
