228 FERGUSON'S LECTURES. 



LECT. 

 vn. 



LECTURE VII. 



OF OPTICS. 



LIGHT consists of an inconceivably great number of 

 particles flowing from a luminous body in all manner of 

 directions ; and these particles are so small, as to sui- 

 pass all human comprehension. 



That the number of particles of light is inconceivably 

 great, appears from the light of a candle ; which, if 

 there be no obstacle in the way to obstruct the passage 

 of its rays, will fill all the space within two miles of the 

 candle every way, with luminous particles, before it has 

 lost the icast sensible part of its substance. 



A ray of light is a. continued stream of these particles, 

 flowing from eve r " visible body in a straight line : and 

 that the particles vhemselves are incomprehensibly small, 

 is manifest from the following experiment. Make a 

 small pin-hole in piece of black paper, and hold the 

 paper upright ov. a table facing a row of candles stand- 

 ing by one another ; then place a sheet of paste-board 

 at a little distance behind the paper, and some of the 

 rays which flow from all the candles through the hole in 

 the paper, will form as many specks of light on the paste- 

 board, as there are candles on the table before the plate : 

 The each speck being as distinct and clear, as if there was only 



"manners one s P ec ^ ^ rom one s ^ n S^ G candle : which shews, that 

 of the the particles of light are exceedingly small, otherwise 

 ofHght. 8 tnev could not pass through the hole from so many 

 different candles without confusion. Dr. Niewentyt has 

 computed, that there flows more than 6,000,000,000,000 

 times as many particles of light from a candle in one 

 second of time, as there are grains of sand in the whole 

 earth, supposing each cubic inch of it to contain 

 1,000,000. 



These particles, by falling directly upon our eyes, 



