colour, instead of reflecting--_absorbs_ all, or the greater part of the rays that fall upon it; and, on the contrary, a body that appears _white_ reflects the greater part of the rays indiscriminately without separating the one from the other. Before proceeding to describe the experiments by which the above results were obtained, it may be proper to give some idea of the form and effects of the _Prism_ by which such experiments are made. This instrument is triangular and straight, and generally about three or four inches long. It is commonly made of white glass, as free as possible from veins and bubbles, and other similar defects, and is solid throughout. Its lateral faces, or sides, should be perfectly plane and of a fine polish. The angle formed by the two faces, one receiving the ray of light that is refracted in the instrument, and the other affording it an issue on its returning into the air, is called the _refracting angle_ of the prism, as ACB, (fig. 31.) The manner in which Newton performed his experiments, and established the discovery to which we have alluded, is as follows. In the window-shutter EG, (fig. 31.) of a dark room, a hole F, was made, of about one third of an inch diameter, and behind it was placed a glass prism ACB, so that the beam of light, SF, proceeding directly from the sun was made to pass through the prism. Before the interposition of the prism, the beam proceeded in a straight line towards T, where it formed a round white spot; but being now bent out of its course by the prism, it formed an oblong image OP, upon the white pasteboard, or screen LM, containing the seven colours marked in the figure--the _red_ being the _least_, and the _violet_ the _most_ refracted from the original direction of the solar beam, ST. This oblong image is called the _prismatic spectrum_. If the refracting angle of the prism ACB, be 64 degrees, and the distance of the pasteboard from the prism about 18 feet, the length of the image OP will be about 10 inches, and the breadth 2 inches. The sides of the spectrum are right lines distinctly bounded, and the ends are semicircular. From this circumstance it is evident that it is still the image of the sun, but elongated by the refractive power of the prism. It is evident from the figure, that since some part of the beam, RO, is refracted much further out of its natural course WT, than some other part of the beam, as WP, the rays towards RO have a much greater disposition to be refracted than those toward WP; and that this disposition arises from the naturally different qualities of those rays, is evident from this consideration, that the refracting angle or power of the prism is the same in regard to the superior part of the beam as to the inferior. [Illustration: _figure 31._]