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.
