72 PHILOSOPHICAL TRANSACTIONS. [aNNO 1770. 



I would by no means be understood to assert, that the particles of light are of a 

 spherical figure. But, whatever their figure may be, I conceive that their size 

 must be so small, that the diagonal of each little solid cannot exceed one millionth 

 of one millionth of an inch, or, at least, that each is capable of being circum- 

 scribed within a sphere of that diameter. To the reasons which are given for 

 making them so small, in the treatise so often referred to, I shall here add 

 another, which may perhaps be more generally convincing: namely, that these 

 bodies must be so minute as to find room to enter in, in swarms, at the pupils 

 of the eyes of the smallest microscopic animals, and not to injure, by their 

 stroke, the very delicate fibres of their optic nerves, nor to lacerate the edges 

 of the uvea; which those that enter near the sides of the perforation, if their 

 figure be not round, must often brush with their angles, as they pass by. 

 Now, if it be granted, that the greatest diagonal of each solid corpuscle of light 

 does not exceed one millionth of one millionth of an inch, or that each is 

 capable of being circumscribed with a sphere of that diameter; then the solid 

 content of a sphere of that diameter is the maximum of the solid content of 

 each corpuscle, and the matter in such a sphere, of due density, is the maximum 

 of the matter in each corpuscle. The maximum therefore of the force of 

 motion in each particle, is the force of a spherule of the size assumed, moving 

 with the known velocity of light; and therefore, be the figure what it will, my 

 conclusion which rests entirely on the maximum of size and matter, will still 

 hold good, unless it can be shown that I have underrated the density of each 

 particle; and even if it could be proved that I have assumed the density too 

 small in the proportion of 1 to 12 thousand times the square of one million, 

 still the general conclusion will not be shaken: for this vast increase of the 

 density will raise the force of motion, in each corpuscle, to no more than that 

 of an iron ball of ^ of an inch diameter, moving one inch in a whole year. 



Again, in the preceding diagram, sb being the diameter of the sun, and /b. 

 Be being each the half of one millionth of one millionth of an inch, the space 

 contained between the spherical surfaces/)^/, egh, is the maximum of the space 

 that the particle of light with their due proportion of interstice can fill, as they 

 start forth from the surface of the sun. For, whatever their figure be, it must 

 be such a one as can be laid between these two spherical surfaces. Now the 

 quantity of matter in this space must not be greater than it would on the 

 hypothesis that each figure was spherical, and the number of spherical particles 

 the greatest possible. Since on that hypothesis, the density of matter, crowded 

 into this space, is vastly too great, to be consistent with the appearances of 

 nature; and consequently a greater density would be inconsistent. Therefore 

 the maximum of the matter, and consequently, if the density of each separate 

 panicle has been rightly assumed, the maximum of the solid content, in each 



