158 HERSCHEL AND HIS WORK 



is broad, falls on a screen placed behind the prism, is 

 the whole band equally heated or equally luminous ? and 

 is the whole sunbeam found decomposed into the colours 

 seen ? Regarding equality of heating and illumination 

 in the various colours, Herschel's experiments made it 

 plain that at the red end there are visible rays, which 

 are hotter than those in any other part of the coloured 

 band or spectrum. The heat he found diminishing as 

 the ref rangibility increases from the red to the violet end. 

 The power of illuminating an object, on the contrary, 

 increases from the red to the orange, from the orange 

 to the yellow, and reaches its greatest intensity be- 

 tween the yellow and the green, after which it rapidly 

 decreases in the blue, more so in the indigo, till it 

 becomes "very deficient in the violet." One of his 

 experiments, by the help of a microscope, was with a 

 guinea: "Red showed four remarkable points: very 

 distinct. Orange, better illuminated: very distinct. 

 Yellow, still better illuminated: very distinct: the 

 points all over the field of view are coloured; some 

 green; some red; some yellow; and some white, en- 

 circled with black about them. Between yellow and 

 green is the maximum of illumination : extremely dis- 

 tinct. Green, as well illuminated as the yellow : very 

 distinct. Blue, much inferior in illumination: very dis- 

 tinct. Indigo, badly illuminated : distinct. Violet, very 

 badly illuminated : I can hardly see the object at all." 

 His second inquiry was, Is a sunbeam passing 

 through a prism and received on a screen behind it 

 represented entirely by the coloured and visible band 

 of the spectrum ? His answer to this question was a 

 distinct no, and a hinted suspicion that the no extended 

 or might extend farther than it was in his power to 



