C 431 ] 
that each is capable of being circumfcribed within 
a fphere of that diameter. To the reafons which are 
given for making them fo fmall, in the treatife fa 
often referred to, I fhall here add another, which may 
perhaps be more generally convincing : namely, that 
thele bodies muft be fo minute as to find room to enter 
in, in fvvarms, at the pupils of the eyes of the fmalleft 
microfcopic animals, and not to injure, by their fcroke, 
the very delicate fibres of their optic nerves, nor to 
lacerate the edges of the uvea ; which thofe that enter 
near the fides of the perforation, if their figure be 
not round, muft often brufh with their angles, as 
they pafs by. Now, if it be granted, that the 
greateft diagonal of each folk! corpufcle of light 
does not exceed one millionth of one millionth of an 
inch, or that each is capable of being circumfcribed 
with a fphere of that diameter; then, the folid con- 
tent of a fphere of that diameter is the maximum of 
the folid content of each corpufcle, and the matter 
in fuch a fphere, of due denfity, is the maximum of 
the matter in each corpufcle. The maximum there- 
fore of the force of motion in each particle, is the 
force of a fpherule of the ft ze aftumed, moving with 
the known velocity of light ; and therefore, be the 
figure what it will, my conclufion (p.420.) which refts- 
entirely on the maximum of fize and matter, will 
ftill hold good, unlefs it can be (hewn that I have 
under-rated the denfity of each particle; and even if 
it could be proved that I have aftumed the denfity too 
fmall, in the proportion of 1 to 12 thoufand times the 
fquare of one million,, ftill the general conclufion. 
will not be fhaken : for this vaft increafe of die 
denfity will raiie the force of motion, in each cor- 
pulcle 
