"Unfit for Vision" 



Much of the light gathered by today's telescopes is invisible. 



By Neil deGrasse Tyson 



Before 1800 the word "light," 

 apart from its use as a verb and 

 an adjective, referred just to 

 visible light. But early that year the 

 English astronomer William Herschel 

 observed some warming that could 

 only have been caused by a form of 

 light invisible to the human eye. 



Already an accomplished observer, 

 I lersi he I had discovered the planet 

 Uranus in 1781 and was now exploring 

 the relation between sunlight, color, 

 and heat. He began by placing a prism 

 in the path of a sunbeam. Nothing new 

 there. Sir Isaac New ton hat! done that 



back in the 1600s, when he also named 

 the familiar seven colors of the visible 

 spectrum: red, orange, yellow, green, 

 blue, indigo, and violet. But Herschel 

 was inquisitive enough to wonder what 

 the temperature of each color might be. 

 So he placed thermometers in various 

 regions of the rainbow and showed, as 

 he suspected, that different colors reg- 

 istered different temperatures. 



Such a discovery would have satisfied 

 most scientists, but Herschel then de- 

 cided to put a thermometer in the dark, 

 unlit area adjacent to the red end of the 

 spectrum. Lo and behold, the temper- 



ature was even higher there than in the 

 red. Herschel had discovered "infra" 

 red light, the band just below red. 



In the first of his four papers on the 

 subject published in the Royal Soci- 

 ety's Philosophical Transactions in 1800, 

 Herschel details every variation he in- 

 vestigated. On page 17 he writes: 



[I] conclude, that the full red falls still short 

 of the maximum of heat; which perhaps lies 

 even a little beyond visible refraction. In this 

 case, radiant heat will at least partly, if not 

 chiefly, consist, if I may be permitted the 

 expression, of invisible light; that is to say, 

 of rays coming from the sun, that have such 

 a momentum as to be unfit for vision. 



Herschel's eyepopper was the astro- 

 nomical equivalent of Antoni van 

 Leeuwenhoek's discoveries with a mi- 

 croscope, beginning in 1 674, of "many 

 very little living animalcules, very pret- 

 tily a-moving" in the smallest drop ot 

 lake water. Other investigators imme- 

 diately took up where Herschel left off. 



22 



NATUKAI. HISTORY June 2006 



