scope, as made by Mr. John Browning, 111, Minories, London, who has attained great celebrity in the manufacture of spectroscopes, shows many of the Fraunhöfer lines in the spectrum, the bright lines of metals and

gases, and the absorption-bands in coloured gases, crystals, and liquids.

Another form of the spectroscope is shown in Fig. 107. It is called the Herschel-Browning direct-vision spectroscope, in which the direct vision is produced by

means of two prisms in which the rays of light are refracted and reflected internally, as shown in Fig. 108, in which A B C represent one of the prisms, s a ray of light falling on the surface of the prism at a, and then refracted to b, whence it is reflected to c, and then again reflected to d, where it is again refracted to the surface of the second prism, by which it is again twice refracted and reflected, as shown by the cut to the right, which represents the arrangement of the two

prisms. This spectroscope is fitted in a flat mahogany or leather case, which can be carried in the pocket, and is, therefore, well adapted for geologists and tourists.

a

b

Fig. 108.

Its dispersion is so considerable as to be capable of dividing the D or sodium line in the solar spectrum with a magnifying power of only 5.

Fig. 109 shows another form of the spectroscope,

having its prism on the top of the stand. The circle is divided into degrees, &c., and reads with a vernier, like a theodolite, or surveyor's transit, by means of which the spectral lines can be accurately determined and mapped. The slit at the extreme end of the left tube.

is furnished with a reflecting prism, which enables the observer to examine and compare two spectra in the field of view at the same time. With slight modifications, this spectroscope can be used for determining the refractive and dispersive powers of solids and liquids.

How to use the Spectroscope.-Screw the telescope A, carrying the knife-edges, or slit, at the small end, into the upright ring fixed on to the divided circle, and the other telescope, в, into the ring attached to the movable index. Now place any common bright light in front of the knife-edges, and while looking through the telescope on the movable index (having first unscrewed the clamping-screw under the circle), turn the telescope with the index round the circle until a bright and continuous spectrum is visible.

To obtain the Bright Lines in the Spectrum given by any substance. Remove the bright flame from the front of the knife-edges, and substitute in its place the flame of a common spirit lamp, or, better still, a gas jet known as Bunsen's burner. Take a piece of platinum wire about the size of a fine sewing-needle, bend the end into a small loop about the eighth of an inch in diameter, fuse a small bead of the substance or salt to be experimented on into the loop, and attaching it to any sort of light stand or support, bring the bead into the front edge of the flame, a little below the level of the knife-edges. If the flame be opposite to the knife-edges on looking through the eye-piece of the telescope, the fixed lines due to the substance will be plainly visible. When minute quantities have to be examined, the substance should be dissolved, and a drop of the solution, instead of a solid bead, should be used on the platinum wire.

The delicacy of this method of analysis is so great that the lines of sodium are visible when a solution is employed which does not contain more than 250000th of a grain of sodium.

To view Fraunhofer's Lines in the Spectrum.-Turn the knife-edges towards a white cloud, and make the slit. formed by the knife-edges very narrow by turning the screw at the side of them. In every instance the focus of the telescope should be adjusted in the ordinary way, by sliding the eye-piece, or draw-tube, until it suits the observer's sight, and distinct vision is obtained. It should be borne in mind that lines at different parts of the spectrum require different adjustments in focussing the telescope.

How two Spectra may be seen at the same time.—The small prism turning on a joint in front of the knifeedges is for the purpose of showing two spectra at the same time. To do this, it must be brought close to the front of the knife-edges. Then one flame must be placed in the position in which the flame of the candle is shown in the small figure, and the other directly in front of the slit. On looking through the telescope as before described, the spectra due to two substances will be seen one above the other.

When the slit is turned towards a bright cloud, and a light is used in the position of a candle flame, the spectrum of any substance may be seen, and compared at the same time with the solar spectrum. In this manner Kirchhoff determined in the solar spectrum the presence of the lines of the greater number of the elements which are believed to exist in the sun.

The Prismatic Analysis of Organic Bodies.--Place a very delicate solution of the substance in a clear glass test-tube, which fix in the small clip attached to a ring

that slips on in front of the knife-edges (see Fig. 110). Upon bringing any bright light in front of the tube, on looking through the telescope, if properly adjusted, a bright spectrum will be seen, interrupted by dark bands or lines, called absorption-bands, or lines due to the substance in solution.

One of the simplest and most interesting experiments of this kind can be made with dilute solutions of madder, port wine, and blood. In these dilute

solutions no difference can be detected with the unassisted eye, but on submitting them in the manner just described to the test of spectrum analysis, very different appearances will be presented. The absorption-bands may, however, be more conveniently examined and accurately investigated by means of the micro-spectroscope shown in Fig. 114.

To map out any Spectrum. Place the eye-piece having cross wires in the telescope with the cross in the direction of an x. Then move the telescope so