Investigation of the Powers of the prismatic Colours to heat and illuminate Objects; with Remarks, that prove the different Refrangibility of radiant Heat. To which is added, an Inquiry into the Method of viewing the Sun advantageously, with Telescopes of large Apertures and high magnifying Powers. By William Herschel. LL.D. F.R.S. Part I. Read March 27, 1800. [Phil. Trans. 1800. p. 255.]
After recommending a cautious circumspection in admitting specious appearances and plausible inferences in our researches both after physical and moral truth, the Doctor acknowledges that a general diflidence of this nature had raised a doubt in his mind, that the power of heating and illuminating objects is not equally distributed among the various coloured rays. This surmise received some confirmation from the different sensations he experienced on viewing the sun with his large telescopes, and through various combinations of differently coloured glasses. With some of these combinations he felt a sensation of heat, though he had but little light; while others gave much light, with scarce any degree of heat. Suspecting hence that perhaps certain coloured rays may be more apt to occasion heat, while others, on the contrary, may be more fit for vision, he resolved to put this conjecture to the test of experiments.
The first set of these experiments relate to the heating power of coloured rays. They were made by admitting successively each differently coloured ray of a prismatic spectrum, through a proper aperture in a pasteboard, on a thermometer Whose bulb was blackened, while another similar thermometer, at a certain distance, showed the temperature of the ambient air. The general results here were, that the temperature, or rather the power of heating of the red ray, is greater than any other, bearing a proportion to that of the green ray as 9 to 4, and to the violet, the least calorific, as 13 to 4.
The next series of experiments was on the illuminating power of coloured rays. These were simply made by viewing through a microscope certain opaque bodies, consisting of minute particles, and illuminated successively by different coloured rays. These substances were red, green, and black paper, a piece of brass, a nail, and a guinea.
The uncommon variegated appearances of the metals, and especially of the iron nails, occasioned by their very minute and differently arranged particles, is here mentioned both as an object of admiration, and as singularly conducive to the purposes of the present inquiry; the greater or smaller number of these particles that became discernible by the different coloured rays affording a kind of scale of comparison which led to the inferences here laid down.
These general inferences are, that the red-making rays are very far from having the illuminating power in any eminent degree; that the orange possesses more of it than the red, and the yellow still more; that the maximum of illumination lies in the brightest yellow or palest green; that the green itself is nearly equally bright with
