18 in. focal length, may be employed. A powerful train of eleven prisms was arranged by Mr. Gassiot; the prisms were hollow, and filled with bisulphide of carbon. It is described in the "Phil. Mag." [4] xxviii. 69.

Mr. Browning has had great experience in the construction of spectroscopes; he made the Kew Observatory spectroscope, furnished with nine glass prisms, another of eleven fluid prisms, which he made for T. P. Gassiot, Esq., and also the spectrum apparatus constructed for William Huggins, Esq., for his important researches on the spectra of the fixed stars; and therefore his directions for the use of the spectroscope are given here.

"Screw the telescope carrying the knife-edges at the small end into the upright ring fixed on to the divided circle, and the other telescope into the ring attached to the movable index. Now place any common bright light exactly 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, still better, a gas jet known as a Bunsen's burner (Fig. 105). Take a piece of platinum wire, about the substance 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 of the platinum wire, 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 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 soild bead, be used on the platinum wire.

FIG. 104. -The Gassiot Spectroscope.

"The delicacy of this method of analysis is very great.

Swan found, in

1857 (Ed. Phil. Trans., vol. xxi., p. 411), that the lines of sodium are visible when a quantity of solution is employed which does not contain more than 1-2,500,000th of a grain of sodium.

"To view Frauenhofer's lines on the solar spectrum, it is only necessary to 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 must be adjusted in the ordinary way, by sliding the draw-tube until it suits the observer's sight, and distinct vision is obtained.

FIG. 105.-A Bunsen Burner, with Ring-stand, supporting the Platinum Wire

"It should be noted that lines at various parts of the spectrum require a different adjustment in focusing the telescope.

"The small prism turning on a joint in front of the knife-edges is for the purpose of showing two spectra in the field of view 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 the 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 the candle flame, the spectrum of any substance may be seen, compared with the solar spectrum. In this manner Kirchoff 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.

PROFESSOR STOKES'S ABSORPTION BANDS.

"The instrument is expressly adapted to the prismatic analysis of organic bodies, according to the method recommended by Professor Stokes, in his lecture at the Chemical Society, printed in the 'Chemical News.'

"To observe these bands it is only necessary to place a very dilute solu

tion of the substance in a test-tube, then fix the test-tube in the small clip attached to a ring, which slips on in front of the knife-edges. Upon bringing any bright light in front of the tube, on looking through the telescope, if the instrument has been properly adjusted, a bright spectrum will be seen, interrupted by the dark bands due to the substance in solution.

"One of the simplest and most interesting experiments of this kind can be made by preparing dilute solutions of madder, port wine, and blood.

"In these very dilute solutions no difference can be detected by the unassisted eye; but on submitting them, in the manner already described, to the test of spectrum analysis, very different appearances will be presented.

"The absorption bands may, however, be most conveniently examined, and accurately investigated, by means of Sorby and Browning's new Microspectroscope."

As will be seen from Fig. 106, it is a very compact piece of apparatus, very ingenious in construction, and consisting of several parts. The prism is contained in a small tube, which can be removed at pleasure. Below the prism is an achromatic eye-piece, having an adjustible slit between the two lenses; the upper lens being furnished with a screw motion to focus the slit. A side

slit, capable of adjustment, admits, when required, a second beam of light from any object whose spectrum it is desired to compare with that of the object placed on the stage of the microscope. This second beam of light strikes against a very small prism suitably placed inside the apparatus, and is reflected up through the compound prism, forming a spectrum in the same field with that obtained from the object on the stage

A is a brass tube carrying the compound direct-vision prism.

B is a milled head, with screw motion to adjust the focus of the achromatic eye-lens.

C, milled head, with screw motion to open or shut the slit vertically. Another screw at right angles to C, and which, from its position, could not be shown in the cut, regulates the slit horizontally. This screw has a larger head, and when once recognized cannot be mistaken for the other.

D D, an apparatus for holding small tube, that the spectrum given by its contents may be compared with that from any other object on the stage.

E, square-headed screw, opening and shutting a slit to admit the quantity of light required to form the second spectrum. Light, entering the round hole near E, strikes against the right-angled prism which we have mentioned as being placed inside the apparatus, and is reflected up through the slit belonging to the compound prism. If any incandescent object is placed in a suitable position with reference to the round hole, its spectrum will be obtained, and will be seen on looking through it.

F shows the position of the field lens of the eye-piece.

G is a tube made to fit the microscope to which the instrument is applied. To use this instrument, insert G, like an eye-piece is in the microscope-tube, taking care that the slit at the top of the eye-piece is in the same direction as the slit below the prism. Screw on to the microscope the object-glass required, and place the object whose spectrum is to be viewed on the stage. Illuminate with stage mirror if transparent, with mirror and Lieberkuhn and dark well if opaque, or by side reflector, bull's-eye, &c. Remove A, and open the slit by means of the milled head, not shown in cut, but which is at right angles to DD. When the slit is sufficiently open, the rest of the apparatus acts like an ordinary eye-piece, and any object can be focused in the usual way. Having focused the object, replace A, and gradually close the slit till a good spectrum is obtained. The spectrum will be much improved by throwing the object a little out of focus.

Every part of the spectrum differs a little from adjacent parts in refrangibility, and delicate bands or lines can only be brought out by accurately focusing their own parts of the spectrum. This can be done by the milled head B. Disappointment will occur in any attempt at delicate investigation, if this direction is not carefully attended to.

When the spectra of very small objects are to be viewed, powers of from in. to 1-20th, or higher, may be employed.

Blood, matter, aniline red, permanganate-of-potash solution (quite fresh), are convenient substances to begin experiments with. Solutions that are too strong are apt to give dark clouds instead of delicate absorption bands.

Mr. Browning makes small cells and other contrivances to hold fluids for examination.

The spectra obtainable from solid, liquid, and gaseous incandescent bodies may be arranged in three orders.

A spectrum of the first order is that which is produced by a solid incan

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