how it is that a body which reflects most of the light that falls upon it appears perfectly white.
But we have not yet done with the variegated band, or prismatic spectrum, as it has been termed. If a highly sensitive finger were held in the yellow rays of the spectrum, a degree of warmth would be felt, greater than if it were held in the violet rays. But if it were removed to the extreme red rays a great deal more heat would be perceived than in either of the former cases. Now, we have imagined the existence of a finger far more sensitive to slight variations of temperature than ordinary fingers are, but these results have been obtained by means of very delicate thermometers, or heat measurers.
Let us now take a piece of paper, prepared for the photographic process, and place it upon the screen so that it may receive the rainbow-like colours upon its sensitive surface. On removing it it will be found to be blackened at a point beyond the violet rays of the spectrum. The principle which blackens the prepared paper is actinism.
From these experiments we learn that the sunbeam is an ethereal band of different rays, which may be separated by the instrumentality of the prism. We learn that heat is less refracted, or bent, by the glass than the other powers, as we find it but slightly thrown out of the right line which the beam would have taken had it not been interrupted by the prism. "We discover that light is subject to greater refraction as the seven colours are thrown
