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a happy contrivance, which it would be out of place here to describe, Dr. Blair succeeded in correcting this secondary spectrum, or in removing the green and claret-coloured fringes which appeared in the best telescopes, and to this contrivance he gave the name of the Aplanatic Telescope.
But while Newton thus overlooked these remarkable properties of the prismatic spectrum, as formed by different bodies, he committed some considerable mistakes in his examination of the spectrum which was under his own immediate examination. It does not seem to have occurred to him that the relations of the coloured spaces must be greatly modified by the angular magnitude of the sun or the luminous body, or aperture from which the spectrum is obtained ; and misled by an apparent analogy between the length of the coloured spaces and the divisions of a musical chord,* he adopted the latter, as representing the proportion of the coloured spaces in every beam of white light. Had two other observers, one situated in Mercury, and the other in Jupiter, studied the prismatic spectrum of the sun by the same instruments, and with the same sagacity as Newton, it is demonstrable that they would have obtained very different results. On account of the apparent magnitude of the sun in Mercury, the observer there would obtain a spectrum entirely without green, having red, orange, and yellow at one end, the white in the middle, and terminated at the other end with blue and violet. The observer in Jupiter would, on the contrary, have obtained a spectrum in which the colours were much more condensed. On the planet Saturn a spectrum exactly similar would have been obtained,
* " This result was obtained," as Newton says, “by an assistant whose eyes were more critical than mine, and who, by right lines drawn across the spectrum, noted the confines of the colours. And this operavon being divers times repeated both on the same and on several papers, I found that the observations agreed well enough with one another." Optics, Part II. Book III.
notwithstanding the greater diminution of the sun's apparent diameter. It may now be asked, which of all these spectra are we to consider as exhibiting the number, and arrangement, and extent of the coloured spaces proper to be adopted as the true analysis of a solar ray.
The spectrum observed by Newton has surely no claim to our notice, merely because it was observed upon the surface of the earth. The spectrum obtained in Mercury affords no analysis at all of the incident beam, the colours being almost all compound, and not homogeneous, and that of Newton is liable to the same objection. Had Newton examined his spectrum under the very same circumstances in winter and in summer, he would have found the analysis of the beam more complete in summer, on account of the diminution of the sun's diameter; and, therefore, we are entitled to say that neither the number nor the extent of the coloured spaces, as given by Newton, are those which belong to homogeneous and uncompounded light.
The spectrum obtained in Jupiter and Saturn is the only one where the analysis is complete, as it is incapable of having its character altered by any farther diminution of the sun's diameter. Hence we are forced to conclude, not only that the number and extent of the primitive homogeneous colours, as given by Newton, are incorrect; but that if he had attempted to analyze some of the primitive tints in the spectrum, he would have found them decidedly composed of heterogeneous rays.
There is one consequence of these observations which is somewhat interesting. A rainbow formed in summer, when the sun's diameter is least, must have its colours more condensed and homogeneous than in winter, when the size of its disk is a maximum, and when the upper or the under limb of the sun is eclipsed, a rainbow formed at that time will lose entirely the yellow rays, and have the green and the red in perfect contact. For the same reason, a rainbow formed in Venus and Mercury will be destitute of green rays, and have a brilliant bow of white light separating two coloured arches; while in Mars, Jupiter, Saturn, and the Georgian planet, the bow will exhibit only four homogeneous colours.
From his analysis of the solar spectrum, Newton concluded,“ that to the same degree of refrangibility ever belonged the same colour, and to the same colour ever belonged the same degree of refrangibility;" and hence he inferred, that red, orange, yellow, green, blue, indigo, and violet were primary and simple colours. He admitted, indeed, that the same colours in specie with these primary ones may be also produced by composition. For a mixture of yellow and blue makes green, and of red and yellow makes orange;" but such compound colours were easily distinguished from the simple colours of the spectrum by the circumstance, that they are always capable of being resolved by the action of the prism into the two colours which compose them.
This view of the composition of the spectrum might have long remained unchallenged, had we not been able to apply to it a new mode of analysis. Though we cannot separate the green rays of the spectrum into yellow and blue by the refraction of prisms, yet if we possessed any substance which had a specific attraction for blue rays, and which stopped them in their course, and allowed the yellow rays to pass, we should thus analyze the green as effectually as if they were separated by refraction. The substance which possesses this property is a purplish blue glass, similar to that of which fingerglasses are made. When we view through a piece of this glass, about the twentieth of an inch thick, a brilliant prismatic spectrum, we find that it has exercised a most extraordinary absorptive action on the different colours which compose it. The red part of the spectrum is divided into two red spacese
separated by an interval entirely devoid of light. Next to the inner red space comes a space of bright yellow, separated from the red by a visible interval. After the yellow comes the green, with an obscure space between them, then follows the blue and the violet, the last of which has suffered little or no diminution. Now it is very obvious, that in this experiment, the blue glass has actually absorbed the red rays, which, when mixed with the yellow on one side, constituted orange, and the blue rays, which, when mixed with the yellow on the other side, constituted green, so that the insulation of the yellow rays thus effected, and the disappearance of the orange, and of the greater part of the green light, proves beyond a doubt that the orange and green colours in the spectrum are compound colours, the former consisting of red and yellow rays, and the latter of yellow and blue rays of the very same refrangibility. If we compare the two red spaces of the spectrum seen through the blue glass with the red space seen without the blue glass, it will be obvious that the red has experienced such an alteration in its tint by the action of the blue glass, as would be effected by the absorption of a small portion of yellow rays; and hence we conclude, that the red of the spectrum contains a slight tinge of yellow, and that the yellow space extends over more than one-half of the spectrum, including the red, orange, yellow, green, and blue spaces.
I have found also that red light exists in the yel. low space, and it is certain that in the violet space red light exists in a state of combination with the
From these and other facts which it would be out of place here to explain, I conclude that the prismatic spectrum consists of three different spectra, viz. red, yellow, and blue, all having the same length, and all overlapping each other. Hence red, yellow, and blue rays of the very same refrangibility coexist at every point of the spec
but the colour at any one point will be that of the predominant ray, and will depend upon the relative distance of the point from the maximum ordinate of the curve which represents the intensity of the light of each of the three spectra.
This structure of the spectrum, which harmo. nizes with the old hypothesis of three simple colours, will be understood from the annexed diagram, where MN is the spectrum of seven colours, all compounded of the three simple ones, red, yellow,
and blue. The ordinates of the curves R, Y,
and B will express the intensities of each colour at different points of the spectrum. At the red extremity M of the spectrum, the pure red is scarcely altered by the very slight intermixture of yellow and blue. Farther on in the red space, the yellow begins to make the red incline to scarlet. It then exists in sufficient quantity to form orange, and, as the red declines, the yellow predominates over the feeble portion of red and blue which are mixed with it. As the yellow decreases in intensity, the increasing blue forms with it a good green, and the blue rising to its maximum speedily overpowers the small portion of yellow and red. When the blue becomes very faint, the red exhibits its influence in converting it into violet, and the yellow ceases