Fig. 21.
Passage of rays through the spectroscope.
S, S´, Slit through which the light falls on the prisms. 1, 2, 3, 4, Prisms in which the rays are dispersed more and more. a, b, Screen receiving the spectrum, of which the seven principal colours are marked.
"Now when the light falls through the open window, or through a round hole or large slit, the images of the hole made by each coloured wave overlap each other very much, and the colours in the spectrum or coloured band are crowded together. But when in the spectroscope we pass the ray of light through a very narrow slit, each coloured image of the upright slit overlaps the next upright image only very little. By using several prisms one after the other (see Fig. 21), these upright coloured lines are separated more and more till we get a very long band or spectrum. Yet, as you know from our experiments with the light of a glowing wire or of molten iron, however much you spread out the light given by a solid or liquid, you can never separate these coloured lines from each other. It is only when you throw the light of a glowing gas or vapour into the slit that you get a few bright lines standing out alone. This is because all the rays of white light are present in glowing solids and liquids, and they follow each other too closely to be separated. But a gas, such as glowing hydrogen for example, gives out only a few separate rays, which, pouring through the slit, throw red, greenish-blue, and dark blue lines on the screen. Thus you have seen the double, orange-yellow sodium line (3, Plate I.) which starts out at once when salt is held in a flame and its light thrown into the spectroscope, and the red line of potassium vapour under the same treatment; and we shall observe these again when we study the coloured lights of the sun and stars."
"We see, then, that the work of our magic glass, the spectroscope, is simply to sift the waves of light, and that these waves, from their colour and their position in the long spectrum, actually tell us what glowing gases have started them on their road. Is not this like magic? I take a substance made of I know not what; I break it up, and, melting it in the intense heat of an electric spark, throw its light into the spectroscope. Then, as I examine this light after it has been spread out by the prisms, I can actually read by unmistakable lines what metals or non-metals it contains. Nay, more; when I catch the light of a star, or even of a faint nebula, in my telescope, and pass it through these prisms, there, written up on the magic-coloured band, I read off the gases which are glowing in that star-sun or star-dust billions of miles away.
"Now, boys, I have let you into the secrets of my five magic glasses—the magnifying-glass, the microscope, the telescope, the photographic camera, and the spectroscope. With these and the help of chemistry you can learn to work all my spells. You can peep into the mysteries of the life of the tiniest being which moves unseen under your feet; you can peer into that vast universe, which we can never visit so long as our bodies hold us down to our little earth; you can make the unseen stars print their spots of light on the paper you hold in your hand, by means of light-waves, which left them hundreds of years ago; or you can sift this light in your spectroscope, and make it tell you what substances were glowing in that star when they were started on their road. All this you can do on one condition, namely, that you seek patiently to know the truth.
"Stories of days long gone by tell us of true magicians and false magicians, and the good or evil they wrought. Of these I know nothing, but I do know this, that the value of the spells you can work with my magic glasses depends entirely upon whether you work patiently, accurately, and honestly. If you make careless, inaccurate experiments, and draw hasty conclusions, you will only do bad work, which it may take others years to undo; but if you question your instruments honestly and carefully, they will answer truly and faithfully. You may make many mistakes, but one experiment will correct the other; and while you are storing up in your own mind knowledge which lifts you far above this little world, or enables you to look deep below the outward surface of life, you may add your little group of facts to the general store, and help to pave the way to such grand discoveries as those of Newton in astronomy, Bunsen and Kirchhoff in spectrum analysis, and Darwin in the world of life."
[1] In our Fig. 18 the distances are inches instead of feet, but the proportions are the same.
[2] Fairyland of Science, Lecture II.; and Short History of Natural Science, chapter xxxiv.