The Micro-spectroscope.
Spectrum analysis has, from its first introduction by Kirschoff in 1859, maintained its fascination over men of science throughout the civilised world. Microscopists, astronomers, and chemists have assigned to the spectroscope a highly important position amongst scientific instruments of research. At quite an early period of its history it appeared to ourselves to promise an extension of the work of the microscope in pathology and microscopy, and second only to that of astronomy and chemistry. The chief hindrances to the use of the spectroscope were, in the early days, of a twofold nature; a widespread, but quite erroneous view of the serious difficulties of employing the instrument, and the want of a first aid to its use.
So valuable a means of research has this process of analysis proved to be, that the discoveries made by the spectroscope appear marvellous. The spectroscope was first made known as a refined instrument for the analysis of light by two Germans, a physicist and a chemist, Kirschoff and Bunsen. In 1860, the latter succeeded in detecting and separating two new alkaline bodies from all other bodies from the waters obtained from the Durkeim springs, less than 0·0002 part of a milligramme of which can be detected by spectrum analysis. It is to the labours of Huggins, Norman Lockyer and others that we are indebted for the wonderful discoveries made in astronomy; and chiefly so to Brewster, Herschel, and Talbot, for showing that certain metals give off light of a high degree of refrangibility; that distinct bands are situated at a distance beyond the last visible violet ray ten times as great as the length of the whole visible spectrum from red to violet.
With regard to the discoveries made in connection with physiological research, we are indebted to F. Hoppe, who in 1862 first described the absorption bands of human blood. His results were confirmed by the investigations of Professor Sir George Gabriel Stokes, who, by adding certain reducing agents to the blood, found that he could change scarlet blood into purple—“purple cruorine”—and in this way the place occupied by the absorption band in the spectrum could be made to change. He reduced the hæmoglobin by robbing the blood of its oxygen. Thus, by Stokes’ and other methods, we have since arrived at extremely valuable results, and the explanation of the difference in colour between arterial and venous blood; and it has also enabled us to show wherein the breathing power of the red corpuscles resides, and further explains phenomena which before his investigations were inexplicable.
Fig. 191.—Fräunhofer’s Spectrum Lines.
The spectroscope seems likely to be of almost as great use in medicine as it has already proved to be in solar and terrestrial chemistry, if we may form an opinion from the large amount of literature which has appeared on the subject. The inception of this magical instrument arose on the instance of a discovery made by Dr. Wollaston in 1802, who, on making a slit in the shutter of his room, instead of a round hole, the spectrum of sunlight, instead of being composed of a number of coloured discs, was now a band of pure colours, each colour being free from admixture with the next to it. Moreover, he found that this colour band was not continuous, as Newton described it, but interrupted here and there by fine black lines.
In 1814, Fräunhofer,[35] a German optician, discovered these lines quite independently, and mapped out 576 of them, calling the more prominent of them A, B, C, D, E, F, G, H, which lines he used as marks of comparison. He also found that the distances of these lines from each other may vary according to the nature of the substance composing the prism; thus, their relative distances are not the same in prisms of flint-glass, crown-glass, and bisulphide of carbon, but they always occupy the same position relatively to the colours of the spectrum. Kirschoff and Angström had mapped out in 1880 no less a number than 2,000 Fräunhofer lines, a portion of which are correctly shown in the accompanying chart ([Fig. 191]).
In 1830, Simms, a London optician, made an improvement in the construction of the spectroscope by placing a lens in front of the prism, so arranged that the slit was in the focus of the lens. This lens turns the light, after it has passed through the slit, into a cylindrical beam before entering the prism. Another lens, also introduced by him, receives the circular beam emerging from the prism, and compels it to throw an image of the slit, which may be magnified at pleasure for each ray. The lens between the prism and the slit is termed the collimating lens. Thus the following are the essential parts of a chemical spectroscope:—(1) a slit, the edges of which are two knife-edges of steel very truly ground, and exactly parallel to each other, and in a direction parallel to the refracting edge of the prism, to admit a pencil of rays. (2) A collimating lens; a convex lens with the slit at its principal focus, which renders the rays parallel before entering the prism. (3) A prism of dense glass, in which the rays are refracted and dispersed. (4) An observing telescope constructed like an astronomical refractor of small size, and placed so that the rays shall traverse it after emerging from the prism. Such are the essentials of a one-prism chemical spectroscope.
The form of instrument in use with the microscope is the “direct vision” spectroscope, consisting of two prisms of flint-glass, placed between three of crown-glass cemented together by Canada balsam; the spectrum being viewed directly by the eye. The earliest constructed form of micro-spectroscope is shown in [Fig. 192], the Browning-Huggins.
It was, however, Mr. Sorby who suggested that the prism should be made of dense flint-glass and of such a form that it could be used in two different positions, and that in one it should give twice the dispersion that it would in the other, but that the angle made by the incident and emergent rays should be the same in both positions.
Fig. 192.—The Browning-Huggins Micro-spectroscope.
Fig. 193.
Fig. 193a.
Figs. 193 and 193a represent prisms of the kind arranged to use in two different positions, i and i′ being the same angle as I and I′.
For most absorption-bands, particularly if faint, the prism should be used in the first position, in which it gives the least dispersion; when greater dispersion is required, so as to separate some particular lines more widely, or to show the spectra of the metals, or Fräunhofer’s lines in the solar spectrum, then the prism must be used as in [Fig. 193]a. This answers well for liquids or transparent objects, but it is, of course, not applicable to opaque objects.
To combine both purposes, some form of direct vision-prisms that maybe applied to the body of the microscope is required. [Fig. 194] represents an arrangement of direct vision-prisms, invented by Herschel. The line R R′ shows the path of a ray of light through the prisms, where it would be seen that the emergent ray R′ is parallel and coincident with the incident ray R.
Fig. 194.
Fig. 194a.
Another very compact combination is shown in [Fig. 194]a. Any number of these prisms (P P P) may be used, according to the amount of dispersion required. They are mounted in a similar way to a Nicol’s prism, and are applied directly over the eye-piece of the microscope. The slit S S is placed in the focus of the first glass (F) if a negative, or below the second glass if a positive eye-piece be employed. One edge of the slit is movable, and, in using the instrument, the slit is first opened wide, so that a clear view of the object is obtained. The part of the object of which the spectrum is to be examined is then made to coincide with the fixed edge of the slit, and the movable edge is screwed up, until a brilliant coloured spectrum is produced. The absorption-bands will then be readily found by slightly altering the focus. This contrivance answers perfectly for opaque objects, without any preparation; and, when desirable, the same prism can be placed below the stage, and a micrometer used in the eye-piece of the microscope, thus avoiding a multiplicity of apparatus.
Fig. 195.—The Sorby-Browning Micro-spectroscopic Eye-piece.
A later and better form of instrument is the Sorby-Browning eye-piece ([Fig. 195]), shown in section ([Fig. 196]) ready for inserting into the body-tube of the microscope, the prism of which is contained in a small tube, removable 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, 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.
Fig. 196.—Sectional view of bright-line Spectroscope; the letters also apply to the standard spectrum scale ([Fig. 198]).
A is a brass tube, carrying the compound direct vision prism; B, a milled head, with screw motion to adjust the focus of the achromatic eye lens C, seen in the sectional view as a triple combination of prisms. Another screw at right angles to C, which from its position cannot be well shown in the figure, regulates the slit horizontally. This screw has a larger head, and when once recognised cannot be mistaken for the other. D D is a clip and ledge for holding a small tube, so that the spectrum given by its contents may be compared with one from an object on the stage. E is a round hole for a square-headed screw, opening and shutting a slit, admitting the quantity of light required to form the second spectrum. A light entering the round hole near E strikes against the right-angled prism, which is placed inside the apparatus, and is reflected up through the slit belonging to the compound prism. If any incandescent object be placed in a suitable position with reference to the round hole, its spectrum will be obtained. F shows the position of the field lens of the eye-piece. The tube is made to fit the microscope to which the instrument is applied. To use this instrument insert F as an eye-piece 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 the stage mirror if it be transparent; with mirror, Lieberkühn, and dark well, by side reflector, or bull’s-eye condenser if opaque. Remove A, and open the slit by means of the milled-head, not shown in figure, but which is at right angles to D D. When the slit is sufficiently open the rest of the apparatus acts as 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.
Sectional View.
Figs. 197 and 197a.—The Beck-Sorby Micro-spectroscope Eye-piece, drawn on a scale of one half size.
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 focussing that particular part of the spectrum. This can be done by the milled-head B. Disappointment will occur in any attempt at delicate investigation if the directions given be not carefully followed out.
Opposite E a small mirror is attached. It is like the mirror below the stage of a microscope, and is mounted in a similar manner. By means of this mirror light may be reflected into the eye-piece, and in this way two spectra may be procured from one lamp.