The next step was one of great scientific importance, indicating, not only the differentiation of the different modes of action in the sun-beam, but the actual separation and isolation of the different agents. This took place just at the opening of the present century. It was shown by Sir William Herschel, in 1800, that, when the sunbeam is decomposed by a glass prism, as shown in Fig. 1, the heat is distributed
Fig. 1.—Positions of the Three Spectra.
unequally through the series of colors—is lowest in the violet, increases in the yellow, but is most intense in the red. This he determined by the use of delicate thermometers, and, in the same way, he proved that the thermal rays of the sunbeam are not all thrown into the visible spectrum, but are of such low refrangibility that they accumulate in the dark space below the red. There is therefore a spectrum of dark rays, producing heating effects, which, beginning at A Fig. 1, increases in strength till it approaches the red, and then fades away in the upper region of the spectrum.
These results of Herschel were followed by the discovery of Ritter, made the next year (1801), that the chemical rays, which had been shown to be most active in the violet portion of the spectrum, were also thrown by refraction into the dark space beyond the violet. As a thermometer was the test in the case of heat, so an appropriate chemical substance has to be used to test the distribution of this force.
