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158 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 refrangibility 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 between 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, encircled with black about them. Between yellow and green is the maximum of illumination: extremely distinct. Green, as well illuminated as the yellow: very distinct. Blue, much inferior in illumination: very distinct. 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