43 The Smithsonian Institution 



void of space from the sun to the earth in its rays, and falling 

 upon us affects our senses in various ways. 



When it falls upon our bodies it produces a sensation of 

 warmth ; when it falls upon our eyes it produces a sensation 

 that we call " light " ; when it falls upon our skin it produces 

 also an effect different from either; for instance, it tans the 

 cheek, by what we call chemical action, but these three dif- 

 ferent effects are caused by the same thing solar energy, 

 which differs in its manifestations according to the body on 

 which it falls, but is 'one and the same always in its essence. 

 When it falls upon the ocean it draws the water up into the 

 sky to drop subsequently to the earth as rain ; when it falls 

 upon the land, it rears everything from the blade of grass to 

 the tree ; and so through all animate and a large part of 

 inanimate nature we find everything that affects man and 

 his interests on the earth to come to us in this sunbeam, 

 whose study gradually leads to conclusions of not merely 

 interesting but of an eminently practical character. 



Sir Isaac Newton, letting these rays pass through a prism, 

 discriminated between them, pointing out that they were com- 

 posed of different colors, but he did not know that there was 

 anything in them beyond what the eye could see. Nearly 

 one hundred years later, in the first year of the present cen- 

 tury, it occurred to Sir William Herschel to move a thermom- 

 eter in the spectrum formed by a prism, and notice the heat 

 in the different rays. He found little heat in the blue, more 

 in the green, and more still in the red, where to the eye the 

 spectrum appears to end. Carrying the thermometer still 

 further, that is, entirely outside and beyond the visible spec- 

 trum at its red end, he found that the instrument rose still 

 more, showing that there was something there invisible to 

 the eye. It was recognized later that the heat in this invisi- 

 ble region was greater than all the heat in the region that 



