Elementary Treatise on Physics

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1877 Excerpt: ...are suitably arranged, the seven reflected pencils may be caused to fall on the ceiling in such a manner as to form seven distinct images--red, orange, yellow, etc. When the mirrors are moved so that the separate images become superposed, a single image is obtained, which is white. v. By means of Newton's disc, fig. 448, it may be shown that the seven colours of the spectrum form white. This is a cardboard disc of about a foot in diameter; the centre and the edges are covered with black paper, while in the space between there are pasted strips of papers of the colours of the spectrum. They proceed from the centre to the circumference, and their relative dimensions and tints are such as to represent five spectra (fig. 449). Fig-448. When this disc is rapidly rotated, the effect is the same as if the retina received simultaneously the impression of the seven colours. vi. If by a mechanical arrangement, a prism, on which the sun's light falls, is made to oscillate rapidly, so that the spectrum also oscillates, the middle of the spectrum appears white. These latter phenomena depend on the physiological fact, that sensation always lasts a little longer than the impression from which it results. If a new impression is allowed to act, before the sensation arising from the former one has ceased, a sensation is obtained consisting of two impressions. And by choosing the time short enough, three, four, or more impressions may be mixed with each other. With a rapid rotation the disc is nearly white. It is not quite so, for the colours cannot be exactly arranged in the same proportions as those in which they exist in the spectrum, and pigmai-555 Colour of Bodies. 471 colours are not pure. A similar explanation applies to the experiment of the oscillating prism. 553. Ne...