3. Graphical method of observing the mode of vibration of a tuning 4. Shells of compressed and rarefied air produced by a source of sound. ΤΟ 15. Copy of a photograph of the spectrum of the spark between poles of zinc and cadmium, showing the separation of the three spectra 16. Image of horizontal arc of sodium vapour thrown on a vertical slit 53 . FIG. 17. Long and short lines of Strontium and Calcium. 18. Arrangement of electric lamp for demonstrating the existence of long and short lines 19. Plan and section of cup used with salts PAGE 54 59 20. Aluminium cup placed in the spark-stand as in use 21. Method of observing the spectra of solutions 22. Examination of the spectra of solution by blowing the solute into the racteristic absorption of some bodies 23. Diagram illustrating the various kinds of absorption, and the cha 64 67 24. Absorption of different thicknesses of the colouring matter of litmus. 69 25. Arrangement for observing the absorption spectra of metallic vapours 70 26. Arrangement for observing the absorption spectra of the metals driven into vapour by the oxhyydrogen flame. 27. Reduced copy of Becquerel's photograph of the complete solar spectrum taken in 1842 28. Wave-length solar spectrum showing the lines (from L to R) the positions of which have been determined by Mascart. 29. The H-lines in the blue end of the solar spectrum, from a photograph by the author 30. Dr. Miller's arrangements for photographing spectra 31. Dr. Miller's diagrams of the ultra-violet spectra of some of the elements in air 32. Copies of Dr. Miller's maps of the ultra-violet spectrum of platinum in various gases showing the length of the solar and ultra-violet spectrum 73 33. Slit allowing five photographs to be taken on the same plate by using 97 38. Arrangement for obtaining and comparing lines with solar spectrum. 103 39. Diagram showing the photographic effect on salts of silver 40. Copy of Professor Tait's photograph of the spark in air 41. Hypothetical arrangement of molecules of oxygen before and after STUDIES IN SPECTRUM ANALYSIS. CHAPTER I. WAVES. § 1. Preliminary. THE work of the true man of Science is a perpetual striving after a better and closer knowledge of the planet on which his lot is cast, and of the universe in the vastness of which that planet is lost. The only way of doing this effectually, is to proceed as gradually, and therefore as surely as possible, along the dim untrodden ground lying beyond the known. Such is scientific work. There is no magic, no fetish in it. There is no special class of men to whom it is given to become more familiar with the beauties and secrets of nature than another. Each of us by his own work and thought, if he so choose, may enlarge the circle of his own knowledge at least, and thus make the universe. more and more beautiful, to himself at all events, if not to others. Futher, it now and then happens in the history of the human race upon this planet, that one particular generation gathers a rich harvest of this better and closer knowledge, this advancement generally coming from an exceeding small germ of thought. Several such instances suggest themselves. How once a Dutchman experimenting with two spectacle-glasses produced the Telescope; and how the field of the known and the knowable has been enlarged by the invention of that wonderful instrument. How once Sir Isaac Newton was in a garden and saw an apple fall; and how the germ of thought which was started in his mind by that simple incident fructified into the theory of Universal Gravitation. Each step of this kind has more firmly knit the universe together, has welded it into a more and more perfect whole, and has enhanced the marvellous beauty of its structure. Future times will say that either this, or perhaps the next, generation, is as favoured a one as that which saw the invention of the telescope or the immortal discovery of Newton for as by the invention of the telescope the universe was almost infinitely extended; as from Newton's discovery we learned that like energies were acting in like manner everywhere; so in our time does the Spectroscope show us that like matter is acting in like manner everywhere; so that if matter and energy be not identical, then these two, namely, matter and energy, may be termed the foundation stones of the universe in which we dwell. The newer the science the more wary must be the |