Chap. 2.] Force or Momentum of Light. 181 



point of the fun's flirface emits one hundred and fifty 

 particles of light in one fecond, we may conclude that 

 this will be fufficient to afford light to the eye with- 

 out any feeming intermiffion ; and yet, fuch is the 

 velocity with which light proceeds, that flill thefe 

 particles will be at leaft one thoufand miles diftant from 

 each, other *. If it was not indeed for this extreme 

 tenuity of the fluid, it would be impoffible that the 

 particles fhould pate, as we know they do, in all di- 

 rections without interfering with each other. In all 

 probability the fplendour of all vifible objects may be 

 in proportion to the greater or lefs number of parti- 

 cles, which are emitted or reflected from their furface 

 in a given fpace of time ; and if we even fuppofe three 

 hundred particles emitted fuccefnvely from the fun's 

 furface in a fingle fecond, ftili thefe particles will follow 

 each other at the immenfe diftance of above five hun-* 

 dred miles. 



That light is, however, not deftitute of FORCE or 

 MOMENTUM; has been proved by the experiment of 

 Mr. Mitchel, already mentioned f. On that experi* 

 ment the following calculation is grounded. If the 

 inftrument weighed ten grains, and the velocity with 

 which it moved was one inch in a fecond, the quantity 

 of matter contained in the rays which fell upon the in- 

 ftrument in that time was equal to the twelve hundred 

 millioneth part of a grain j the velocity of light ex- 

 ceeding the velocity with which the inftrument moved 

 in that proportion. The light in this experiment was 

 collected from a furface of about three fquare feet, 

 which reflecting qnly half what falls upon it, the quan- 

 tity of matter contained in the rays of the fun incident 

 upon a fquare foot and half of furface is no more than 



* Priellley's Optics, p. 385. t See p. 95. 



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