Ligttt. 135 



substance is lost by the luminous body. If the 

 whole space were filled with men, every eye would 

 see the candle the moment it was posited in a 

 visible situation. Farther, how small must the 

 particles of light be, when they pass without re- 

 moving the minutest particles of microscopic dust 

 that lie in their way, and even these minute par- 

 ticles are rendered visible, by reflecting back the 

 particles of light that strike against them ! 



Small as the particles of light are, it is highly 

 probable that, though diffused through all space, 

 they are separated from each other by distances 

 of much more than a thousand miles. This may 

 be inferred as follows : It is an obvious fact, that 

 the effect of light upon our eyes is not instantane- 

 ous, but that the impression remains for some 

 time. You may easily satisfy yourselves of this, 

 by shutting your eyes after having looked for 

 some time on a candle, a star, or any other lumi- 

 nous body, when you will perceive that a faint 

 picture will remain of the object for some time. 

 The smallest division of time, that we can well 

 conceive, will be the one hundred and fiftieth 

 part of a second. If, therefore, one lucid part of 

 the sun's surface emits one hundred and fifty par- 

 ticles of light in a second of time, we may con- 

 ceive that these will be amply sufficient to afford 

 light to the eye without any intermission. You 

 will remember, then, that light travels at the rate 

 of about one hundred and seventy thousand miles 

 in a second ; so that, the sun emitting one hun- 



