Page:The American Cyclopædia (1879) Volume V.djvu/100

COLOR orange, yellow, green, blue, indigo, and violet. The proof that these are the elements of white light was first furnished by the experiments of Sir Isaac Newton, in 1672. It must be added, however, that between any two of the simple colors of the spectrum there is a gradual inter- change of hue, so that in fact the different colored rays existing in and obtainable from the white ray are not seven, but indefinite in number. The same sensible hue of certain colors which, when found in the spectrum, cannot be further decomposed, can always be separated by the prism when found in natural objects, giving two or more, but not all of the spectral colors; that is, the colors of natural objects are always compound. Light from white paper gives a complete spectrum ; from yellow, a fragment of a spectrum, showing the yellow space bright, with some green and or- ange, sometimes a faint tinge additionally of blue and red ; blue paper gives a whole spec- trum, less the orange and red ; and so of others. The light of the purest yellow flower, analyzed, shows also some green and orange. On account of the impossibility of obtaining pure pigments, a colored solution or glass transmits not only the rays that make up its own color, but other colors that *in the spec- trum are neighboring to these. Light coming through a blue glass is found to contain a strong blue element, feebler green and indigo, and still feebler yellow and violet. Looking through the blue glass at a spectrum, the same colors are seen, and in corresponding degrees of vividness. The blue medium passes through itself also the closely related rays, but it ex- tinguishes or cuts off the more distant. Bodies vary in color with their mechanical division. Light when reflected from the surface of gold is yellow, and when transmitted through thin leaves of this metal is green ; but when gold is reduced to a fine powder and suspended in water, the mixture will transmit, according to the state of division, blue, purple, or ruby light, as was shown by Faraday. Finely di- vided sulphur ordinarily transmits red light, but on precipitating it from a solution of sul- phydric acid, and adding sesquichloride of iron, the liquid will for a short time transmit purple light. By decomposing sulphydric acid in so- lution with heat, the water holding the sus- pended sulphur appears blue. The theory now commonly received is that proposed by Sir. David Brewster, namely, that when a body appears permanently of any given color, simple or compound, it is because it absorbs the re- maining colors of the spectrum, i. e., of the complete or white ray, and reflects those which constitute the color of which it appears. Thus a scarlet flower is a body having such relations to light that it absorbs violet, blue, &c., re- flecting only certain proportions of red, orange, and yellow. In this view, bodies, as snow and milk, appear white when they reflect or trans- mit entire or white light; others, as certain inks and coal, appear black, because they ab- sorb all the colored rays in the proportions which form white light. But as all bodies both absorb and reflect in some degree, the proportion of light which white bodies absorb and black bodies reflect is also a mixture of the various colors in the proportions forming white. To prove that the most intensely black body still reflects some light, look through a long narrow tube into a room completely dark- ened ; this may be considered as presenting before the eye an absolutely black spot. Let a piece of coal or black velvet be brought against the inner end of the tube ; the body is at once visible, as something differing clearly from the black space. Oersted separated *he light coming from bodies into two parts. First, he found that all bodies have in some degree the property of mirrors, but that in those not polished the light is reflected from so many small surfaces in various directions that no image is produced. Secondly, he found that illuminated bodies gave out another por- tion of light not reflected, which had the prop- er color of the bodies themselves. A beam of light, in passing through certain media, or in being reflected from a surface at a certain angle, has all its vibrations reduced to a com- mon plane, and is then said to be polarized. (See LIGHT.) Arago proved that from illumi- nated unpolished bodies a small and usually imperceptible amount of light having the color of the luminary is returned to the eye, and that this light is polarized by reflection ; while much the larger portion of their light, having the color of which the body is seen, is polar- ized by refraction, and hence must have come from beneath the surface of the bodies emitting it. A theory Jong since proposed in sub- stance by Euler is explained at length in his " Letters to a German Princess." Two viols or other stringed instruments being attuned in unison, though many feet apart, if a string of one be made to give out a musical sound, the corresponding string of the other, being fitted to vibrate in the same times, takes up the vibrations from the air, and actually emits the same sound, or some chord of that sound. The first induces vibrations in the second ; the sec- ond reciprocates the vibrations of the first ; but the conditions of the second must first be suit- able. In this view, when sunlight falls upon various bodies, it induces (according to the undulatory theory, by the communication of agitation from the ethereal medium to the mo- lecules of the bodies) a luminous condition or power in the illuminated body, but which in most bodies remains only so long as the action of the original luminary is exerted upon them. But though the body receives the whole impul- sion of white light, it will in turn reproduce the whole, or such part as the condition of its molecules fits it for reciprocating. If it recipro- cate all the colored rays in due proportion, and in considerable amount, it will be white ; if in like proportion, but in almost imperceptible amount, it will be called black. If a body re-