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Rh still transmitted considerably more light than the red glasses, they remedied the former inconvenience of an irritation arising from heat. Repeating these trials several times, I constantly found the same result." How to see the sun distinctly without inconvenience or danger from the heat continued to occupy his thoughts for years. "I viewed the sun through water," he wrote in 1801. "It keeps the heat off so well, that we may look for any length of time, without the least inconvenience." "Ink diluted with water gave an image of the sun as white as snow; and I saw objects very distinctly, without darkening glasses."

Herschel introduced his papers on the sun's light and heat with a wise remark, which proved him to be as good an observer in the world of mind as in that of matter. "It is sometimes of great use in natural philosophy to doubt of things that are commonly taken for granted; especially as the means of resolving any doubt, when once it is entertained, are often within our reach. . . . It will therefore not be amiss to notice what gave rise to a surmise, that the power of heating and illuminating objects might not be equally distributed among the variously coloured rays." The experiments, which he then made on the light and heat given out by each colour of the spectrum, were admirably imagined and beautifully carried out. He was really engaged on a continuation of Newton's experiments on sunbeams, but the field of research was new and untrod. Gradually the questions to which he sought answers began to take shape more distinctly in his mind. When a prism intercepts a beam of sunlight, let into a darkened room through a hole in the window shutter, and the band of coloured light, five times as long as it