PAPER. making the (1.) Into a piece of hard glass combustion-tube, thoroughly FOURTH cleaned and closed at one end, a few pieces of metallic sodium, clean and as free as possible from naphtha, were introduced. The end of the tube was then drawn out and Method of connected with a Sprengel pump and exhausted as rapidly as possible. Hydrogen was then admitted, and the tube re-exhausted, and, when the pressure was again reduced to a few millimetres, carefully scaled up. The tube thus prepared was placed between the slit plate of a spectroscope and a source of light giving a continuous spectrum. Generally, unless the atmosphere of the laboratory was very still and free from dust, the two bright D lines could be distinctly seen on the background of the bright continuous spectrum. experiment. The D The tube containing the sodium was then heated with a Bunsen flame, and the spectrum carefully watched. Soon after the application of the heat, a dark line, thin and delicate as a spider's thread, was observed to be slowly creeping down each of the bright sodium lines and exactly Occupying the centre of each. Next, this thin black line was observed to thicken at the top where the spectrum of the lower denser vapours was observed, and to advance downwards along the D line, until arriving at the bottom. they both became black throughout; and if now the heat lines re was still applied, thus increasing the density of the various layers of the sodium vapour, the lines began to broaden until, in spite of considerable dispersion, the two lines blended into one. The source of heat being now removed, the same changes occurred in inverse order; the broad band split into two lines, gradually the black thread alone was left, and finally that vanished, and the two bright lines were restored. (2.) This experiment was then varied in the following way-Some pieces of metallic sodium were introduced into a test-tube, and a long glass tube conveying coal-gas passed to the bottom, an exit for the gas being also provided at the top. The sodium was now heated and the versed. FOURTH PAPER. flow of coal-gas stopped. In a short time the reversal of the D lines was complete. The gas was now admitted, The black and a small quantity only had passed when the black lines were reduced to threads. lines thin out when the vapour In my former communications to the Royal Society I is diluted. have pointed out the extreme importance of these facts in connection with solar and stellar physics. In observing the sun by the new method, we get various Fraunhofer lines thickened in the spots and thinned in the chromosphere and prominences; and in these latter, in some instances, notably in the case of F, we find the lines gradually widening as they approach the limb of the sun. A section of the While this may be remarked as a solar demonstration of the correctness of the conclusion at which Dr. Frankland and myself had arrived, it is to be noted that bright line prominences may occasionally be seen on the sun's disc over or near spots in the spectrum of which the same lines are thick, while this phenomenon could not exist if the thickening of the lines were due to temperature alone. METHOD EMPLOYED.1 The method of observing spectra to which I have. already referred, and which has been adopted in the work of which I now propose to give an account, consists in throwing an image of the spark on the slit of a spectroscope in the laboratory experiments in exactly the same manner in which I proposed, in 1866, that an image of the sun should be thrown on the slit in order to spectroscopically examine minute portions of the sun and his surrounding atmosphere. It is obvious that in this method the image of the slit will be associated in the spectroscope with an image of a spark is observed. section of the spark; and that if from any cause there be 1 This method was first exhibited at a lecture at the Royal Institution April 2nd. 1870. The same method has more recently been employed with great success by M. Salet in a research on the spectra of the Metalloids. various shells of vapour surrounding each pole, which shells give different spectra, then these spectra will be sorted out so that their variations may be traced from pole to pole. The arrangements adopted will be easily gathered from the annexed woodcut (Fig. 150) and the accompanying C FOURTH PAPER. FIG. 150.-A, collimator; B, ob‹erving-telescope; c, spark; D, lens. description. It is scarcely necessary to add that an important condition of this new method is that the objectglass of the collimator should be filled with light, and also that no light should be wasted. So long as these conditions. obtain, conjugate foci and different lenses may be employed FOURTH Spectroscope. Coil. Photographic arrange and the size of the image varied at pleasure, and still the brightness of the spectrum will be sufficient. The instruments with which the observations have been made are as follows:--A large spectroscope, a sister instrument to that used by Bunsen and Kirchhoff in their celebrated researches, and made by the same maker, Steinheil, of Munich. It is furnished with four prisms of flint glass. Three are of an angle of 45° and one of 60°. The general arrangements of the instruments are described. by Kirchhoff in his memoir. In front of the slit plate is placed a lens throwing on the slit the image of the spark. A coil, made by Apps, and giving a 4-inch spark. A large Leyden jar has also been occasionally used as a condenser on the secondary wire. Beneath the observing-telescope is placed a commutator, by which the current is controlled by the observer without changing his position. The window of my laboratory looks due south, and the collimator is placed in the same direction; and when it became necessary to have the solar spectrum in the field, the light reflected from a heliostat placed outside the laboratory in direct prolongation of the line of collimation was thrown on to the lens and thus on to the slit, where the size and intensity of the images could be varied at pleasure by altering the position of the lens. When it was required to photograph a spectrum, the ordinary observing-telescope of the spectroscope was disments. mounted, and its place supplied by a telescope of 3 inches aperture and 49 in. focus. This was supported on the cast-iron table of the spectroscope at one end, and at the other on a stand. The eyepiece and its mounting were removed, and against the end of the tube, thus left free, a small camera-box holding a plate 4 in. by 3 in. This spectroscope has been temporarily placed at my disposal by Professor Guthrie, of the Royal School of Mines, to whom my best thanks are due. |