6 g THE SUN. 



ference." Every ray of light which comes from the sun is 

 not a simple but a compound thing. Here, again, I must 

 explain. The air we breathe is not a simple but a com- 

 pound thin*. It is separable at least into four distinct 

 things, as different from one another as any four things 

 you can name. Well, then, so of a ray or beam of the 

 sun ; it may be separated, split, subdivided, not into four, 

 but into many hundreds, nay, thousands, of perfectly dis- 

 tinct rays or things, or rather of three distinct sorts or 

 species of rays ; of which one sort affects the eyes as 

 light; one the eense of feeling and the thermometer as 

 heat \ and one the chemical composition of everything 

 it falls upon ; and which produces all the effects of photo- 

 graphy. Each of these three classes (and I believe there 

 are several more, indeed I have proved the existence ot 

 one more) consists of absolutely innumerable species or 

 sorts ; every one of which is separated from every other 

 by a boundary line, as sharp and as distinct as that which 

 separates Kent and Sussex on a map. A ray of light is 

 a world in miniature, and if I were to set down all that 

 experiment has revealed to us of its nature and constitu- 

 tion, it would take more volumes than there are pages 

 in the manuscript of this lecture. 



(29.) When the sun's light is allowed to pass through 

 a small hole in a dark place, the course of the ray or 

 sunbeam maybe traced through the air (by reason of the 

 small fine dust that is always floating in it), as a straight 

 line or thread of light of the same apparent size, or very 

 nearly so, from the hole to the opposite wall. But if in 

 the course of such abeam, be neld at any point tU edge 



