158 LIGHT AND COLOURS. [LeiSOn XXV. 



were not surprisingly small, it woul i strike against 

 bodies with great force : this would be very inju- 

 rious to those tender organs the eyes, and would 

 probablv soon causcvhlindness : but it is found that 

 these particles are small, almost beyond concep- 

 tion ; for it has b en computed that there fly out 

 of the end of a flame of a burning candle in a se- 

 cond or time, ten thousand nrnlliuns of timer- more 

 such panicles than there are visible grains of sand 

 in the whole earth : it they were not inconceivably 

 small, it is cer'ain, therefore, that the flame would 

 be entire!) dissipated and lost. 



It is also asserted, that the panicles of Light are 

 emitted in right lines : preserving their rectili- 

 near motion till they are turned out of their path 

 by some of the following causes, viz. either by the 

 attraction of some other body near which they pass, 

 which is called inflection} or, by passing obliquely 

 through a body of different density, which is called 

 refraction ; or, by being turned aside by the op- 

 position of some intervening body, \vhi( h is called 

 reflection; or, lastly, by b- ing totally stopped by 

 some body into which they penetrate, and this is 

 called their extinction. A succession of these par- 

 ticles following one another, in an exact right line, 

 is called a ray of Light ; and this ray, in whatever 

 manner its direction mav be changed, whether by 

 reflection, refraction, or inflection, always preserves 

 a rectilinear course, till it be again changed. It is 

 to be observed, that bodies, as they respect the rays 

 of light, are divided into three kinds : K Those 

 which emit the rays of Light; as the sun and fixed 



stars : 



