From: The Spectroscope: its uses in general analytical chemistry by T. T. Baker, 1907

The Comparison Spectroscope. — This is an instrument designed for roughly comparing the absorption spectra oftwo fluids or coloured translucent objects. A very convenient form is made by Zeiss, the principle of which is seen in Fig. 26. Here F is the stage of the instrument, with two openings, Gl and G2, which are illuminated (as in a microscope) by two mirrors (not shown) underneath the stage. Over these openings are placed cells containing the fluids to be compared, and the light from each is reflected, by means of the special prisms R1 and R2, into the slit S. The eye is placed at C, and it sees the two spectra side by side—i.e., one above the other—whilst a photographed scale of wave-lengths is also thrown on to the spectra by means of the side-tube, which is illuminated at D. Zeiss also makes now a similar instrument for the simultaneous examination of three spectra. A noteworthy accessory is a cell, which is provided with a micrometer arrangement for varying the height of the coloured liquid; this is useful when comparing two absorption spectra; the movement reads to '05 millimetre—i.e., the height of the column of liquid through which the light passes before entering the slit can be measured to '05 millimetre. The metal parts of this ' absorption vessel,' which come in contact with liquid, are of nickel.

The preliminary adjustment of the instrument is carried out as follows. By turning the milled ring B in Fig. 26 the lens O is focussed until the bright lines in a known spectrum (e.g., the Fraunhofer lines in daylight or the lines from a vacuum-tube spectrum) are seen quite distinctly. The width of the slit is regulated by the screw A. The wavelength scale can then be brought into perfect adjustment with the lines and locked in position. The two prisms R1 and R2 are provided with lenses L1 and L2, whose focal length (measured in glass) is equal to their distance from the slit; hence only parallel rays enter the slit.

By means of such a comparison spectroscope the position and limits of absorptions may be conveniently measured by illuminating one-half of the slit with a source of illumination giving several known lines, and the other half of the slit with the coloured substance under examination. The scale need not, therefore, necessarily be used.