ON SPECTRUM ANALYSIS. LECTURE I. Introduction.-Newton's Discovery of the Composition of White Light, 1675.-Properties of Sun Light.-Heating Rays.-Luminous Rays.-Chemically Active Rays.-The Solar Spectrum.Position of Maxima.-Illustrations of these Radiations.-Means of obtaining a Pure Spectrum.-Fraunhofer's Lines.-Planet and Moon Light.-Star Light. APPENDIX A.-Extracts from "Newton's Opticks." APPENDIX B.-Burning Magnesium Wire a Source of Light for photographic purposes. APPENDIX C.-On the Chemical Action of the constituent parts of Solar Light. AMONGST all the discoveries of modern science none has deservedly attracted more attention, or called forth more general admiration, than the results of the application of Spectrum Analysis to chemistry. Nor is this to be wondered at when we remember that a new power has thus been placed in the hands of the chemist, enabling him to detect the presence of chemical substances with a degree of delicacy and accuracy hitherto unheard of, and thus to obtain a far more intimate knowledge of the composition of terrestrial matter than he formerly B enjoyed. So valuable a means of research has this new process of analysis proved itself to be that since its first establishment, some seven short years ago, no less than four new chemical elements have by its help been discovered. Not only, however, have we to consider the importance and interest which attaches to the subject as evidenced by the discovery of these new elementary bodies, but we are forced to admit that by the application of the simple principles of spectrum analysis the chemist is able to overstep the narrow bounds of our planet, and, extending his intellectual powers into almost unlimited space, to determine, with as great a degree of certainty as appertains to any conclusion in physical science, the chemical composition of the atmosphere of the sun and far distant fixed stars. Nay, he has even succeeded in penetrating into the nature of those mysteries of astronomy, the nebulæ; and has ascertained not only the chemical composition, but likewise the physical condition, of these most distant bodies. It does, indeed, appear marvellous that we are now able to state with certainty, as the logical sequence of exact observations, that bodies common enough on this earth are present in the atmosphere of the sun, at a distance of ninety-one millions of miles, and still more extraordinary that in the stars the existence of such metals as iron and sodium should be ascertained beyond a shadow of doubt. We thus see that the range of inquiry which the subject opens out is indeed vast, and it is well to bear in mind that as the discoveries in this branch of science are so recent, they are necessarily incomplete, so that we must expect to meet with many facts and observations which still stand alone, and require further investigation to bring them into harmony with the rest. The advance in these new fields of research is, however, so rapid that, as time rolls on, and our range of knowledge widens, new facts quickly come to support the hitherto unexplained phenomena, and thus our theory becomes more and more complete. It will be my duty in the Lectures which I have the honour of delivering in this Hall to endeavour to explain to you that these results, apparently as marvellous as the discovery of the elixir vitæ or the philosopher's stone, are the plain and necessary deductions from exact and laborious experiment, and to show how two German philosophers, quietly working in their laboratories in Heidelberg, obtained this startling insight into the processes of creation. The only means of communication which we possess with the sun, planets, or far distant stars, or by which we can ascertain anything respecting their chemical constitution, is by means of the life-supporting radiation which they pour down upon the earth, producing the effects which we call light and heat. It will, therefore, be our business, in the first place, to investigate the composition of the radiations which these bodies give off, and next gradually to notice, as our field of observation enlarges, the applications to which the properties. of the light thus emitted lead us. One cannot help regretting that when, as at present, the sunlight is shining so brightly, we are unable to utilize it, and illustrate by experiments made with the sunlight itself the points which we wish to explain. In this climate, however, even if we could conveniently do it, the sun shines so intermittently, and it is so doubtful if we can have it just when it is required, that we have to make use of other means, especially of this bright light of the electric arc, which, if less perfect than sunlight, is more under our control. In the year 1675, Sir Isaac Newton presented to the Royal Society his memorable treatise on Optics.1 In this treatise, which contains a large number of experimental and theoretical investigations, one point especially attracts our attention it is the discovery of the decomposition of white light. We have here (Fig. 1) a facsimile of the drawing illustrating Newton's experiment on this subject. He heads the first paragraph in his memoir, Р K B FIG. 1. written in the year 1675, with the words, "lights which differ in colour differ also in refrangibility." Newton allowed the sun to shine through a round hole (Fig. 1, F) in a shutter, and he then examined the character of this light by means of a triangular piece of glass (A B C) called a prism. He found that the white light, after passing through the prism, was bent or refracted out of its course, and split up into a coloured band (PT) which, when received on the white screen (M N), exhibited all the colours of the rainbow in regular succession, passing from red through all the shades of orange, yellow, 1 For extracts from "Newton's Opticks," see Appendix A. green, and blue, to violet. Newton termed this coloured band the Solar Spectrum, and came to the conclusion that the light of the sun consists of rays of different degrees of refrangibility. He also showed that all the various portions of this coloured band, when again brought together, produce upon the eye the effect of white light. This experiment (represented in Fig. 2), Newton performed by simply allowing the light passing through the round hole (F) in the shutter to fall on a prism (A B C), producing the solar spectrum, and then, S 30 a FIG. 2. on looking at this coloured band through another prism (abc) placed in the same direction, instead of seeing a coloured band he observed a spot of white light, thus showing that the whole of these differently coloured rays, when brought together by means of the second prism, produce on the eye the effect of white light. Here we have the intensely white electric light, and by means of these two prisms you observe that I can split the light up into its various constituent parts, and we obtain this splendid coloured band, the spectrum of the electric arc. Now I shall endeavour to show you the |