respecting the Nat^ire of Light. 199 



thi,?^ by supposing a small instant of time to take place between 

 the emission of each particle of light that passes off in the same 

 direction; which is by no means a crude supposition ; for, as 

 the impression of light does certainly remain for a short space 

 of time on the retina, it is easy to conceive that there must be 

 an interval between two particles of light entering the eve ; for, 

 if the impression of the second particle of light were to affect the 

 eye before the first had ceased, a confused vision must ensue, 

 from that organ being affected with severul impressions at the 

 same time. Chevalier D'Arcy has attempted to prove that the 

 sensation of a particle of light will remain for the seventh part 

 of a second; and this, by computing the velocity with which 

 light moves, will give an interval of 20,000 miles between two 

 particles of light. But as I have not seen the original paper 

 written by D'Arcy, which is to be found in the Acadernv of 

 Sciences for 1765, I do not know upon what experiments he 

 makes the above estimation. That the impression lasts the in- 

 terval he mentions is a correct estimate, I am inclined to think, 

 from an experiment I made with a stick having its point made 

 red hot. When the stick made less than eight revolutions in 

 one second, the circle of light was not perfect, but appeared to 

 consist of a series of lumiiious points extremely near to each 

 other. 



I have stated above, that the impression of light according 

 to D'Arcy continues for the seventii part of a second, which 

 is taken from Dr. Mutton's Mathematical Dictionary. But Dr. 

 Brewster in his edition of Ferguson's Astronomy, vol. ii., savs, 

 " It appears from the accurate experiments of D'Arc)', that the 

 impression of light upon, the retina continues two tninutes and 

 forty seconds." Wliich of these two respectable writers is right, 

 respecting the estimation made by D'Arcy, I am unable to state j 

 but it appears to me, from the experiment just mentioned, that 

 Dr. Button's is tiie most correct statement. We may also 

 observe, if the impression remained for so long a time as two 

 minutes and forty seconds, that upon shutting out the light in 

 the day-time, or extinguishing a lighted body, a room would 

 not appear so instantaneously dark, but objects would remain for 

 a short time visible. 



The second o!ijcctio)i is, that if there be .such a constant 

 emission of particles from luminous bodies, thev would in asiiort 

 time be much wasted, and therefore the stars ought to decrease 

 f.iid the iiulk of llie planets increase. This oi)jection Dr. Hors- 

 ley has endeavoured to remove; by first proving that the sim 

 and stars receive mutually a great quantity of light from each 

 other d rectly, and from thy j)lanets by reflection. He bv nia- 

 ihcmatical calculation shows that in the space of 385,130,000 

 N 4 years 



