68 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 thing. 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 sense 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 of 

 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 may be 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 a beam, be held at any point the edge 



