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Spectrum Analysis: Browning

Mr. Browning has produced a lantern by which the spectrum can be thrown upon a screen for lecture purposes. It is furnished with are electric lamp, the light from which is projected through a narrow slit on to a prism. Indeed, the apparatus is founded upon Newton's experiment, but is so far improved that it is quite independent of sunlight. Moreover, the bright lines due to sodium and other metals can be shown by its aid. The reversal of the sodium line from dark to light is such a favourite experiment, and it so well illustrates the use of the spectroscope, that I will quote Mr. Browning's directions concerning it :-- "The experiment is one which requires a little skill in manipulation, yet it is not difficult if the following precautions are attended to. Let the lamp be carefully closed in with a cover of stiff brown paper over the apertures left for the purpose of ventilation ; set light to a piece of metallic sodium, the size of a pea, in a small iron spoon by means of a spirit-lamp. This must be done inside the lantern, the spoon being first made nearly red-hot. The spoon should be attached to a small holder, and left inside the lamp with the door shut, until the lamp becomes filled with the vapour of burning sodium. One of the carbon crucibles or the lower rod of the electric lamp having had a piece of sodium placed upon it, the upper rod of the electric lamp can be brought into contact with it; the intense light generated will then produce a faint continuous spectrum on the screen, with a very bright sodium line predominant. After a short time, varying between a few seconds and two or three minutes, if the lantern has been sufficiently filled with the vapour of the sodium burning in the spoon, these bright lines will become dark ones. The explanation of this experiment is that the vapour of the sodium in the lantern being cooler than the sodium burning between the poles of the lamp, the bright line of the sodium is absorbed by the sodium vapour.

The most simple form of spectroscope, as well as the easiest to use, is that shown in the annexed cut, "The Miniature Spectroscope." It consists of a compound direct-vision prism placed in a sliding tube, at the end of which is a lens. At the end of the instrument furthest from the eye-piece is an adjustible slit, shown in the cut at the right-hand side. The two jaws or knife edges constituting this slit can be made to approach or recede from one another by turning the milled ring on the outer part of the tube. When viewing the Fraunhofer lines, the slit is made as narrow as possible, but it can with advantage be opened wider for the lines of chemical spectra. In viewing the bright lines given by any substance, a spirit lamp, or, better still, a Bunsen burner, is placed opposite the slit. Such a burner is shown in the next figure. At the base of the burner are some holes to admit air, and these can be opened or closed by turning a ring at the bottom of the tube. When open, the air mixes freely with the gas, and the burner gives a blue flame suitable for spectroscope work. At the top of the lampstand will be seen a holder with a piece of platinum wire attached, which projects into the flame. Upon a loop at the end of this wire is fused a bead of any substance or salt to be submitted to experiment. If the quantity of the substance is very small, it can be dissolved, and a drop of the solution placed in the platinum loop. The substances used should be of the purest possible kind, for the least impurity may render the experiment futile. This can be readily appreciated when it is known that this method of analysis is so delicate that the lines of sodium can be detected in a quantity of solution containing only the 1/2500000 of a grain of that metal. Sodium indeed seems to be an element of such abundance that it is almost impossible to get rid of its traces so far as the spectroscope is considered. A little dust, or the touch of a finger, is quite sufficient under any circumstances to give the sodium line. Bodies, whether solid, liquid, or gaseous, are said to be coloured when they absorb certain rays and reflect others-white when they reflect all colours, and black when they absorb all, reflecting none. A very important branch of spectroscopic work is in the observation of what are known as absorption phenomena. In these observations we no longer study the light given out by bodies, except by interposing between that light and the spectroscope certain substances which absorb part and transmit part. In transparent solids, such as coloured glasses, we place them against the slit of the instrument to observe the rays which are shut out. In observing the spectra of liquids, a tube or cell is used. Thus, supposing that we wish to examine the reddish-violet solution of permanganate of potash, better known under the name of "Condy's fluid," we place it in a tube or cell against the slit of the spectroscope, when we shall find that parts of the red, blue, and violet are transmitted without hindrance, while the yellow and green parts of the spectram are occupied by black bands. In this way, solutions which appear to the unaided eye to be identical-such as the one just named, madder, and blood-show such differences under the spectroscope that their nature can be readily detected. In this way it may be imagined how a blood stain may be examined and identified, and how the spectroscope could be made available in criminal cases. The instrument, for the same reason, is of extreme importance to medical examinations generally, for it will detect the presence of blood in secretions of the body where it has no business to be. A very elegant method of viewing absorption spectra, and comparing them the one with the other, has been furnished by Mr. Browning. A plate of twelve substances in gelatine is comprised in a space about two inches square. Its nature can be understood from reference to the annexed cut, the substances in this instance being dyes.

For spectrum analysis of very minute bodies, or particular parts of such bodies, the micro-spectroscope is the best instrument to employ. This wonderful piece of apparatus, as perfected by Messrs. Sorby and Browning, is figured in the next cut. It will plainly indicate the most minute quantity of blood, adulterations in various articles of food, as well as the absorption bands in the leaves and juices of plants. I am indebted to Mr. Browning for the following description of the instrument.

The prism is contained in a small tube, A, which can be removed at pleasure. Below the prism is an achromatic eye-piece, having an, adjustable slit between the two lenses; the upper lens being furnished with a screw motion to focus the slit. A side slit, capable of adjustment, adjusts, 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, the brass tube carrying the compound direct-vision prism, has a sliding arrangement for roughly focussing. B, a milled head, with screw motion to finally adjust the focus of the achromatic eye-lens. c, milled head, with screw motion to open or shut the slit vertically. Another screw, II, at right angles to c, regulates the slit horizontally. This screw has a larger head, and, when once recognised, 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 the other object on the stage. E, a screw, opening and shutting a slit to admit the quantity of light required to form the second spectrum. Light entering the aperture 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 aperture, it 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 in the microscope tube. 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, H, at right angles to D D. When the slit is sufficiently open, the rest of the apparatus acts like an ordinary eye-piece, and any object can be focussed in the usual way. Having focussed 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 slightly from adjacent parts in refrangibility, and delicate hands or lines can only be brought out by accurately focussing their own parts of the spectrum. This can be done by the milled head, n. 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 half-inch to one-eighth may be employed.