with a Conjecture, fuggefting a Mean, &c. 197 
the quantity of matter in the body, will, in the refult, exprefs 
its force or momentum. 
If, then, we can fuppofe the quantity of matter in a particle 
of light to be, not indeed abfolutely, but comparatively, o, its 
momentum will alfo be comparatively o ; and it can have, in 
that cafe, no vifible effect on the fmalleft particle of duft to 
remove it. 
Let us now confider what reafon there is for fuch a fuppofi- 
tion. In order to that, I beg leave to introduce here, a para- 
graph from one of my letters to Dr. Franklin, printed with 
his letters and papers on philofophical fubjeéts. It runs thus,* 
** The flame of a candle, it is faid, may be feen four miles 
round. The light, diffufed through this circle of eight miles 
diameter, was contained, before it left the candle, within a 
circle of half an inch diameter. If the-denfity of light, in 
thefe circumftances, be as thofe circles to each other, that is, 
as the fquares of their diameters (or, which is equivalent, if 
the denfity decreafes as the fquare of the diftance or femi-di- 
ameter increafes) the candle-light, when come to the eye, will 
be 1027,709,337,600 times rarer than when it firft quitted the 
half-inch circle. Now the aperture of the eye, through which 
the light paffes, does not exceed one-tenth of an inch diameter, 
and the portion of the lefs circle, which correfponds to this 
fmall portion of the greater circle, muft be proportionably, 
that is, 1027,709,337,600 times lefs than one-tenth of an 
inch : and yet this infinitely {mall point (if you will allow the 
expreffion) affords light enough to make it vifible : or rather, 
iffords light fufficient to affect the fight at that diftance.” 
If 
» Letters; &c. P 275. 
