13 



in the same strait course as before. They may likewise 

 he contracted into a less, .or diffused through a larger 

 space, while the quantity of light continues the same, 

 neither eacreased nor diminished. So in the focus of 

 a burning glass, all tSie rays which would otherwise pass 

 directly through the glass, are contracted into one bright 

 spot, while the circumambient space for the breadth of 

 the glass is deprived of its light and left shaded : and 

 the action of light, thus condensed, is proportional t*> 

 its quantity, and produces all the effects of the most in- 

 tense fire, yea, such as no culinary fire will produce. 

 Whence it is plain that fire and light are essentially 

 the same, and that fire is only condensed light* 



The materiality of light is farther confirmed by its 

 motion; for vision is propagated through this medium 

 successively, as sound is through air. This has l>een de- 

 monstrated from the eclipses of Jupiter's satellites ; for 

 the satellite having been hid behind the planet, it re- 

 quires a certain time after it emerges before its light can 

 reach the eye ; namely, seven minutes and a half, which 

 is a motion six hundred thousand times swifter than that 

 of sound through the air. 



The quantity of elementary light is c&teris parilm, 

 every where the same at the same distance from the 

 sun: but it faction is more er less intense, as the rays 

 are more direct or oblique. These are in a continual 

 vibrating motion, going and returning to and from the re- 

 sisting medium, in exceeding short and imperceptible in- 

 tervals, which makes the element seem to be at perfect 

 rest. All the rass are refracted and reflected alternately ; 

 so that the same incident ray, which is refracted at one 

 interval, is reflected at the next : this is visible in trans- 

 parent mediums, where the rays fall upon glass, water, 

 and the i;kt ; but in opake bodies, though the fact is the 

 same it is noi so sensibl . When the rays fail upc*n 

 glass, they are reflected one moment and transmitted 

 the next: an 1 this vibrating motion seems to be essen- 

 tial to light, when Us rays are put into motion* 

 B 6 



