Scientific American 8, 14.11.1846
In the year 1666 Sir, Isaac Newton began to investigate the subject of the variety of colors and the immediate cause of their appearance; and finding the colored image of the sun formed by a glass prism, to be of an oblong, and not of a circular form, as according to the laws of refraction, it ought to be, he was surprised at the great disproportion between its length and breadth, the former being five times the length of the latter; and he began to conjecture that light is not homogeneal, but that it consists of rays, some of which are much more refrangible than others. Prior to this period, philosophers supposed that all light, in passing out of one medium into another of different density, was equally refracted, in the same or like circumstances; but that there are different species of light, and that each species is disposed both to suffer a different degree of refrangibility in passing out of one medium into another, and to excite in us the idea of different color from the rest; and that bodies appear of that color which arises from the peculiar rays they are disposed to reflect. It is now, therefore, universally acknowledged that the light of the sun, which to us seems perfectly homogeneal and while, is composed of no fewer than seven different colors, namely, Red, Orange, Yellow, Green, Blue, Indigo, and Violet. A body which appears of a red color has the property of reflecting the red rays more powerfully than any of the others: a body of a green color reflects the green ray more copiously than rays of any other color, and so of the orange, yellow, blue, purple and violet. A body which is of a black color, 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 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 A. C. B. The manner in which Newman performed his experiments, and established the discovery to which we have alluded, is as follors:
In the window-shutter, E G 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 A C B, so that the beam of light S F, 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 upon the white pasteboard, or screen, L M, containing the seven colors, marked in the figure, the red bring the least, and the violet the most refracted from the original direction of the solar beam, S T. This oblong image is called the prismatic spectrum. If the refracting angle of the prism, A C B, be 64 degrees, and the distance of the paste-board from the prism about 18 feet, the length of the image will be about ten inches, and the breadth 2 inches. The side of the spectrum are right lines distinctly bounded, and the ends are semi-circular. From the circumference, it is evident that it is still the image of the sun, but elongated by the refractive power of the prism.
By making a hole in the screen, L M, opposite any one of the colors of the spectrum, so as to allow that color alone to pass — and by letting the color thus separated fall upon a second prism — Newton found that the light of each of the colors was alike refrangible, because the second prism could not separate them into an oblong image, or into any other color. Hence he called all the seven colors simple or homogeneous, in opposition to white light, which he called compound, or heterogenou With the prism which this philosopjer used, he found the lengths of the colors and spaces of the spectrum to be al follows: - Red, 45; Orange, 27; Yellow, 40; Green, 60; Blue, 68(?), Indigo, 45; Violet, 80; or 360 in all. But these spaces vary a little with prisms formed of different substances, and, as they are not separated by distinct limits, it is difficult to obtain anything like an accurate measure of their relative extents. Newton examined the ratio between the sines of incidence and refraction of these decompounded rays, and found that each of the seven primary colors making rays had certain limits within which they were confined. Thus, let the sine of incidence in glass be divided into 50 equal parts, the sine of refraction into air of the least refrangible, and the most refrangible rays will contain respectively 77 and 78 such parts. — The sines of retraction of all the degrees of red will have the intermediate degrees of magnitude, from 77 to 77 one-eighth; Orange from 77 one-eighth to 77 one-fifth; Yellow, from 77 one-fifth to 77 one-third; Green, from 77 one-third to 77 one-half; Blue, from 77 one-half to 77 two-thirds; Indigo, from 77 two-thirds to 77 seven-ninths; and Violet, from 77 seven-ninths to 78.
From what has been stated, it is evident that, in proportion as any portion of an optic glass bears a resemblance to the form of prism, the component rays that pass through it must be necessarily separated, and will consequently paint or tinge the object with colors. The edges of very convex lens approach to this form, and it is on this account that the extremeties of objects, when viewed through them, are found to be tinged with the prismatic colors.
From what has been stated in reference to the solar spectrum, it will evidently appear that white light is nothing else than a compound of all the prismatic colors; and this may he still further illustrated by showing that seven primary colors, when again put together, recompose white light. This may be rudely proved, for the purpose of illustration, by mixing together seven different powders, having the colors and proportion of the spectrum but the best mode; on the whole, is the following: Let two circles be drawn on a smooth round board. covered with white paper, as in figure2; let the outermost be divided into 360 equal parts; then draw seven right lines, as A, B, C, &c., from the centre to the outermost circle, making the lines A and B include 50 degrees of that circle. The lines B and C, 40 degrees; C and D 60; D and E, 60; E and F, 48; F and G. 27; G and A, 45. Then between these two circles paint space A G red, inclining to orange near G; GF orange, inclining to yellow near F; F R yellow, inclining to green near E; E D green, inclining to blue near D; D C blue, inclining to indigo near C; C B indigo, inclining to violet near B; and B A violet, inclining to a soft red near A. This done, paint all thut part of the board black which lies within the inner circle; - and, putting an axis through the centre of the board, let it be turned swiftly round that axis, so that the rays proceeding from the above colors may be all blended and mixed together in coming to the eye. Then the whole colored part will appear like a white ring little grayish - not perfectly white, because no art can prepare or lay on perfect colors, in all their delicate shades, as found in the real spectrum.
That all the colors of light, when blended together in their proper proportions, produce a pure white, is rendered certain by the following experiment: Take a large convex glass and place it in the room of the paper or screen on which the solar spectrum was depicted; the glass will unite all the rays which come from the prism, if a paper is placed to receive therm, and you will see a circular spot of pure lively white. The rays will cross each other in the focus of the glass, and if the paper be removed a little farther from that point, you will set the prismatic colors again displayed, but in an inverted order, owning to the cruising of the rays.