instead of being polychromatic-that is to say, compounded CHAP. IX. of all these beautiful colours-were monochromatic, or of Effect of one colour only, the whole expanse of creation would put monochroon a very different appearance from what it does. If, matic light. instead of illuminating a diagram, the letters of which are of different bright colours, by the white light of the electric lamp, we illuminate it by a light that only contains one colour-by the yellow light of sodium, for instance-and then look at it, we see that some of the letters upon it are almost invisible, whilst others are very clear, the yellow light only allowing a difference to be seen of more or less depth of shade-there being no difference in colour. But if we allow the polychromatic light of an electric lamp or of the sun to fall upon the diagram, we at once see all the letters in different colours. This experiment feebly indicates the advantage we possess in living in a universe lit by white, or polychromatic, or many-coloured light, instead of light which is merely blue, or yellow, or any other single colour. Hitherto we have spoken of refraction. We will now introduce the word dispersion, which represents simply a Dispersion. measure of different refractions, or the difference between the bending of the red and the violet rays of light. In an ordinary spectrum, the difference between the red and the violet is the difference of the refractions of those two colours by the prism; and the angle which the red, or yellow, or other colour, forms with the original path of the Angle of compound-beam, is called the angle of deviation of that deviation. colour. There is one other consideration which we owe to Newton. In his very first experiments, that great philosopher discovered that the quality of the spectrum depended very much on the following consideration:-If we wish to get the best possible effect out of a prism, and the purest possible spectrum, we have so to arrange it that the particular ray which we wish to observe, whether the yellow, the blue, the green, or any other, leaves that prism L CHAP. IX. at exactly the same angle as the incident compound ray falls on it. This angle is termed the angle of minimum Angle of minimum deviation. deviation. the slit in 1802. It is very curious, however, that Newton, although he made many experiments on prisms, really omitted one of the most important points; and here again we get an idea of the enormous patience which is necessary in these matters, for we had to wait a century and a quarter before the next essential point was hit upon which has helped us in our study of the solar spectrum. Newton made a round or oblong hole in a shutter for his experiments, but we now know he ought not to have done that; he ought to have made a slit. But this did not come out until 1802, Wollaston when Dr. Wollaston, by merely using a slit instead of a first uses round hole, made a tremendous step in advance-the real basis of all the modern work which has been done in solar physics by means of the spectroscope. The importance of this is obvious, Suppose we take a cylindrical beam of sunlight and put a prism in the path of the beam, we observe that the spectrum is not a pure one; but if we change the round hole for a slit, we obtain a spectrum of the greatest purity: the red, blue, green, and violet, instead of overlapping and destroying the beauty of the spectrum, show distinctly as simple colours, each one speaking for itself on the screen. By using this narrow slit instead of the round hole which Newton made in the shutter, we got the first idea of the tremendous importance. of spectrum analysis; for no sooner had Dr. Wollaston examined the sunlight with the new arrangement, as Newton had done a century and a quarter before with the old one, than he found out that it was not at all as Newton had represented it. Newton told us, in fact, that the sunlight was continuous; that is to say, that the spectrum was one in which there was no break in the light which flowed out to every part of the spectrum, from the extreme red to the violet. When Dr. Wollaston tried the slit, he found, however, that the spectrum, instead of being an unbroken rainbow band of light, was really broken by a succession of CHAP. IX. fine-beautifully fine-black lines. Here is an extract from Wollaston's communication to the Royal Society: 1 "I cannot conclude these observations on dispersion without remarking that the colours into which a beam of white light is separated by refraction appear to me to be neither seven, as they usually are seen in the rainbow, nor reducible by any means (that I can find) to three, as some persons have conceived; but that, by employing a very narrow pencil of light, four primary divisions of the prismatic spectrum may be seen with a degree of distinctness that I believe has not been described nor observed before. "If a beam of daylight be admitted into a dark room by a crevice of an inch broad, and received by the eye, at the distance of ten or twelve feet, through a prism of flint glass free from veins, held near the eye, the beam is seen to be separated into the four following colours only: red, yellowish-green, blue, and violet; in the proportions represented in Fig. 43. The line A that bounds the red side Wollas ton's paper. A B -C D E F16 43 -The first observation of Fraunhofer's lines. of the spectrum is somewhat confused, which seemed in part owing to want of power in the eye to converge red light. The line B, between red and green, in a certain position of the prism, is perfectly distinct ; so also are D and E, the two limits of violet; but c, the limit of green and blue, is not so clearly marked as the rest and there are also, on each side of this limit, other distinct dark lines, f and g, either of which in an imperfect experiment might be mistaken for the boundary of these colours. The position of the prism in which the colours are most clearly divided is when the incident light makes about equal angies with two of its sides. I then found that the spaces A B, BC, (D, DE, occupied by them, were nearly as the numbers 16 23, 36 25. Since the proportions of these colours to each other have been supposed by Dr. Blair to vary according to the medium by which they are produced, I have compared with this appearance the coloured images caused by prismatic vessels containing substances supposed by him to differ most in this respect, such as strong but colourless 1 Philosophical Transactions, 1802, part i. p. 378. CHAP. IX. Wollas ton's paper. Fraunhofer's paper. nitric acid, rectified oil of turpentine, very pale oil of sassafras, and Canada balsam, also nearly colourless. With each of these I have found the same arrangement of these four colours, and, in similar positions of the prisms, as nearly as I could judge, the same proportions of them. 66 But when the inclination of any prism is altered so as to increase the dispersion of the colours, the proportions of them to each other are then also changed, so that the spaces A C CD, instead of being, as before, 39 and 61, may be found altered as far as 42 and 48. "By candle-light a different set of appearances may be distinguished. When a very narrow line of the blue light at the lower part of the flame is examined alone, in the same manner, through a prism, the spectrum, instead of appearing a series of lights of different hues contiguous, may be seen divided into five images at a distance from each other. The first is broad red terminated by a bright line of yellow, the second and third are both green, the fourth and fifth are blue, the last of which appears to correspond with the division of blue and violet in the solar spectrum and the line D of Fig. 43. "When the object viewed is a blue line of electric light, I have found the spectrum to be also separated into several images, but the phenomena are somewhat different from the preceding. It is, however, needless to describe minutely appearances which vary according to the brilliancy of the light, and which I cannot undertake to explain." Although these lines were observed by Dr. Wollaston, it was not until 1814 that we find them mapped out with the greatest care, to the number of 576, by a German optician named Fraunhofer, whose work was quite independent of Wollaston's; hence they are termed "Fraunhofer lines," the principal ones being lettered A, B, C, &c. Fraunhofer's work will be gathered from the following extract from his communication to the Munich Academy:1 "Into a dark room, and through a vertical aperture in the windowshutter, about 15′′ broad and 36" high, I introduced the rays of the sun upon a prism of flint-glass placed upon the theodolite; this instrument was 24 feet from the window, and the angle of the prism was nearly 60°. The prism was placed before the object-glass of the telescope so that the angles of incidence and emergence were equal. In looking at this spectrum for the bright line which I had found in a spectrum of artificial light, I discovered, instead of this line, an infinite number of vertical lines of different thicknesses. These lines are darker than the rest of the spectrum, and some of them appear entirely black. When the prism was turned so that the angle of incidence increased, these lines disappeared, and the same thing Denkschriften der K. Acad. der Wissenschaften zu München, 1814-15, Band 5, pp. 193-226. Translated in Edinburgh Philosophical Journal, vol. ix. p. 296, and vol. x. p. 26, 1823. happened when the angle was diminished. If the telescope was considerably shortened, these lines reappeared at a greater angle of incidence; and at a smaller angle of incidence the eye-glass required to be pulled much farther out in order to perceive the lines. If the eve-glass had the position proper for seeing distinctly the lines in the red space, it was necessary to push it in to see the lines in the violet space. If the aperture by which the rays entered was enlarged, the finest lines were not easily seen, and they disappeared entirely when it was about 40". "If it exceeded a minute, the largest lines could scarcely be seen. The distances of these lines and their relative proportions suffered no hange, either by changing the aperture in the shutter, or varying the distance of the theodolite. The refracting medium of which the prism is made, and the size of its angle, did not prevent the lines from being always seen. They only became stronger or weaker, and were consequently more or less easily distinguished in proportion to the size of the spectrum. The proportion even of these lines to one another appeared to be the same for all refracting substances; so that one line is found only in the blue, another only in the red, and hence it is easy to recognize those which we are observing. The spectrum formed by the ordinary and extraordinary pencils of calcareous spar, exhibited the same lines. The strongest lines do not bound the different colours of the spectrum, for the same colour is almost always found on both sides of a line, and the transition from one colour to another is scarcely sensible. Fig. 44 shows the spectrum with the lines such as they are actually observed. It is, however, impossible to express on this scale all the lines and the modifications of their size. At the point A the red nearly terminates, and the violet at 1. On either side we cannot define with certainty the limits of these colours, which, however, appear more distinctly in the red than in the violet. If the light of an illuminated cloud falls through the aperture on the prism, the spectrum appears to be bounded on one side between G and H, and on the other at B: the light of the sun, too, of great intensity, and reflected by a heliostate, lengthens the spectrum almost one-half. In order, however, to observe this great elongation, the light between C and G must not reach the eye, because the impression of that which comes from the extremities of the spectrum is so weak as to be extinguished by that of the middle of the spectrum. At A we observe distinctly a welldefined line. This, however, is not the boundary of the red, which still extends beyond it. At a there is a mass of lines forming together a band darker than the adjacent parts. The line at B is very distinct, and of a considerable thickness. From C to D may be reckoned nine very delicate and well-defined lines. The line at c is broad and black like D. Between C and D are found nearly thirty very fine lines, which, however, with the exception of two, cannot be perceived but with a high magnified power and with prisms of great dispersion; they are besides well-defined. The same is the case with the lines between B and C. The line D consists of two strong lines separated by a bright one. Between D and E we recognize about eighty-four lines of different sizes; that at E consists of several lines, of which the middle one is the strongest. From 1 to 7 there are nearly twenty-four lines; at CHAP. IX. Fraunhofer's tafer. |