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.
Zeiss’New Comparison Spectroscope.—This instrument is intended to assist botanists and physiologists in the study of such coloured substances as chlorophyll, and is so named because its special feature is to render possible an exact comparison of the absorption spectra of solutions. As will be seen from the accompanying figuro (fig. 85), the apparatus has the general form of a Microscope, and so far resembles one that the special spectral arrangement can be applied to a Microscope stand. The object-table bears two orifices, 4 cm. apart, through which two mirrors reflect the sun or lamplight perpendicularly upwards. Each of these two pencils, passing through prisms situated in the horizontal box, becomes decomposed and forms a spectrum. The two spectra appear in close proximity, and can be observed through the broad slit C in the eye-piece.
In the side tube D is placed the wave-length scale, which is illuminated by the mirror at the tube’s mouth, its image being projected between the two spectra. The width of tho slit, and consequently the brightness of the spectra, can be altered by the turning of the knob A.
Rotation of the tube-piece B brings out sharp presentation of the scale and spectra ; the screw E causes a side movement of the scale image. The two plane and concave mirrors are movable round vertical and horizontal axes. When the necessary mirror adjustments have been made, two spectra with the most important of Fraunhofer’s lines are seen over one another. It is easy by movement of the scale to get the D line on a = 589. When daylight is not available, a spirit-lamp with a salted wick or asbestos thread soaked in NaCl solution may be used. When direct sunlight is used, the number of Fraunhofer’s lines that appear is endless. The cross-piece carrying the prisms and superincumbent ocular slides up and down in a vertical axis, and can be clamped at any desired height.
For comparative observation of absorption spectra of various solutions it is best to use Zeiss’ double absorption vessels. The management of the instrument is easy and remarkably convenient, and its great feature is its adaptability with even relatively unfavourable light.
