German-British astronomer, composer and brother (1738-1822) of fellow astronomer Caroline Herschel; he discovered Uranus on March 13, 1781, pioneered the use of astronomical spectrophotometry, discovered infrared radiation, and was the first president of the Royal Astronomical Society when it was founded in 1820. Rare ALS signed “Wm. Herschel,” one page, 7 x 8.75, March 18, 1800. Letter written from "Slough near Windsor," in full: "I beg leave to trouble you with the enclosed short paper, which may stand as a post-script or appendix to the one I had the honour of sending you last Friday. The whole process of the invisible solar rays having been completely discovered I judged it expedient not to delay a communication of it; and hope the novelty of the subject will plead my excuse for troubling you with it so soon after my last. I shall be highly obliged by a line to know whether this, and the former parcel, which contained also some wool from Dr. Parry of Bath, have been safely delivered.” Affixed to a slightly larger cardstock mount. In very good to fine condition, with some creasing, a few small edge chips, and moderately heavy overall toning. HerscheI’s discovery of ‘infra-red’ solar rays assured his place in history as a physicist as well as an astronomer. Before 1800, when he presented his theories on the laws of the reflection, refraction, and transmission of radiant heat to the Royal Society, no one had suspected such variations within the spectrum. Herschel's studies also dealt with the question of the chemical properties of prismatic colors.
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