Popular Scientific Recreations in Natural Philosphy, Astronomy, Geology, Chemistry, etc., etc., etc. Gaston Tissandier

may give out through certain media, and we can photograph the spectrum given out under all conditions. The distance of the source of the light makes no difference. So whether it be the sun, or a far-distant star, we can tell by the light sent to us what the physical condition of the star may be. It was discovered in 1864 that the same metallic body may give different spectra; for instance, the spectrum might be a band of light—like the rainbow—or a few isolated colours; or again, certain detached lines in groups. The brightness of the spectrum lines will change with the depth of the light-giving source, or matter which produces it.

      We have become aware by means of the Spectroscope that numerous metals known to us on earth are in combustion in the sun, and new ones have thus been discovered. In the immense ocean of gas surrounding the sun there are twenty-two elements as given by Mr. Lockyer, including iron, sodium, nickel, barium, zinc, lead, calcium, cobalt, hydrogen, potassium, cadmium, uranium, strontium, etc. Not only is the visible spectrum capable of minute examination, but, as in the case of the heat spectrum already mentioned when speaking of Calorescence, the light spectrum has been traced and photographed far beyond the dark space after the blue and violet rays, seven times longer than the visible solar spectrum—a spectrum of light invisible to our finite vision. Although a telescope has been invented for the examination of these “ultra violet” rays, no human eye can see them. But—and here science comes in—when a photographic plate is put in place of the eye, the tiniest star can be seen and defined. Even the Spectroscope at length fails, because light at such limits has been held to be “too coarse-grained for our purposes”! “Light,” says a writer on this subject “we can then no longer regard as made of smooth rays; we have to take into account—and to our annoyance—the fact that its ‘long levelled rules’ are rippled, and its texture, as it were, loose woven”!

      Twenty years ago Professors Kirchkoff and Bunsen applied Fraunhofer’s method to the examination of coloured flames of various substances, and since then we have been continually investigating the subject; yet much remains to be learnt of Spectrum Analysis, and Spectroscopy has still much to reveal. From Newton’s time to the present our scientists have been slowly but surely examining with the Spectroscope the composition of spectra, and the Spectroscope is now the greatest assistant we possess.

      “Spectrum Analysis, then, teaches us the great fact that solids and liquids give out continuous spectra, and vapours and gases give out discontinuous spectra instead of an unbroken light” (Lockyer). We have found out that the sunlight and moonlight are identical, that the moon gives us spectrum like a reflection of the former, but has no atmosphere, and that comets are but gases or vapours. The most minute particles of a grain of any substance can be detected to the millionth fraction. The 1/1000 of a grain of blood can be very readily distinguished in a stain after years have passed. The very year of a certain vintage of wine has been told by means of “absorption,” or the action of different bodies on light in the spectrum. It is now easy, “by means of the absorption of different vapours and different substances held in solution, to determine not only what the absorbers really are, but also to detect a minute quantity.” The application of this theory is due to Dr. Gladstone, who used hollow prisms filled with certain substances, and so thickened the “absorption lines.” By these lines, or bands, with the aid of the Spectrum Microscope, most wonderful discoveries have been made, and will continue to be made. We will close this portion of the subject with a brief description of the Spectroscope in principle.

      The instrument consists of two telescopes arranged with two object-glasses on a stand (fig. 150). A narrow slit is put in place of the eye-piece of one, the arrangement admitting of the slit being made smaller or larger by means of screws. The glass to which the slit is attached is called the collimating lens. The light, at the end of the slit seen from the other telescope, being separated by the prisms between the two telescopes, will produce the spectrum. The Spectroscope is enclosed, so that no exterior light shall interfere with the spectra the student wishes to observe. This merely indicates the principle, not the details, of the Spectroscope, which vary in different instruments.

      We may now pass from the Spectroscope to the Telescope and the Microscope, instruments to which we are most largely indebted for our knowledge of our surroundings in earth, air, and water.

      The word Telescope is derived from the Greek tele, far, and skopein, to see; and the instrument is based upon the property possessed by a convex lens or concave mirror, of converging to a focus the rays of light falling on it from any object, and at that point or focus forming an image of the object. The following diagram will illustrate this. Let VW be a lens, and AB an object between the glass, and F the focus. The ray, Ac, is so refracted as to appear to come from a. The ray from b likewise appears in a similar way, and a magnified image, ab, is the result (fig. 152).

      The ordinary Telescope consists of an object-glass and an eye-lens, with two intermediates to bring the object into an erect position. A lens brings it near to us, and a magnifier enlarges it for inspection. We will now give a short history of the Telescope and its improved construction.

      Roger Bacon was supposed to have had some knowledge of the Telescope, for in 1551 it was written: “Great talke there is of a glass he made at Oxford, in which men see things that were don.” But a little later, Baptista Porta found out the power of the convex lens to bring objects “nearer.” It was, however, according to the old tale, quite by an accident that the Telescope was discovered about the year 1608.

      Fig. 152.—Converging rays to a focus.

      In Middleburg, in Holland, lived a spectacle-maker named Zachary Jansen, and his sons, when playing with the lenses in the shop, happened to fix two of them at the proper distance, and then to look through both. To the astonishment of the boys, they perceived an inverted image of the church weathercock much nearer and much larger than usual. They at once told their father what they had seen. He fixed the glasses in a tube, and having satisfied himself that his sons were correct, thought little more about the matter. This is the story as told, but there is little doubt that for the first Telescope the world was indebted either to Hans Lippersheim or Joseph Adriansz, the former a spectacle-maker of Middleburg; and in October 1608, Lippersheim presented to the Government three instruments, with which he “could see things at a distance.” Jansen came after this. The report of the invention soon spread, and Galileo, who was then in Venice, eagerly seized upon the idea, and returning to Padua with some lenses, he managed to construct a telescope, and began to study the heavens. This was in 1609. Galileo’s Tube became celebrated, and all the first telescopes were made with the concave eye-lens. Rheita, a monk, made a binocular telescope, as now used in our opera- and field-glasses approximately.

      But the prismatic colours which showed themselves in the early telescopes were not got rid of, nor was it till 1729 that Hall, by studying the mechanism of the eye, managed a combination of lenses free from colour. Ten years before (in 1718) Hadley had established the Reflector Telescope; Herschel made his celebrated forty-foot “reflector” in 1789.

      Fig. 153.—The Microscope.

      However, to resume. In 1747, Euler declared that it was quite possible to construct an arrangement of lenses so as to obtain a colourless image, but he was at first challenged by John Dollond. The latter, however, was afterwards induced to make experiments with prisms of crown and flint glass. He then tried lenses, and with a concave lens of flint, and a convex lens of crown, he corrected the colours. The question of proper curvature was finally settled, and the “Achromatic” Telescope became an accomplished fact.

      There are two classes of Telescopes—the reflecting and refracting. Lord Rosse’s is an instance of the former. Mr. Grubb’s immense instrument in Dublin is a refractor.

      The Microscope has been also attributed to Zacharias Jansen, and Drebbel, in 1619, possessed the instrument in London, but it was of little or no use. The lens invented by Hall, as already mentioned, gave an impetus to the Microscope. In the simple Microscope the objects are seen directly through the lens or lenses acting as one. The compound instrument is composed