Sorby-Browning Microspectroscope with bright-line micrometer, c. 1880
Microspectroscope shown mounted on a microscope by Thomas Cooke, York.
While this instrument is unsigned by the maker, it is identical in all respects to examples signed John Browning, London.
The following is from Spectrum analysis in its
application to terrestrial substances by
H. Schellen, 1872.
The
Sorby-Browning Microspectroscope.
The object of this instrument is to facilitate the
accurate observation of the absorptive phenomena of
the smallest solid and liquid bodies, such as are
prepared for microscopic examination a corpuscule of
blood, for instance. Sorby, with the assistance of
Browning, has so arranged the spectroscopic part of
the instrument that it can be applied to any
microscope by fixing it in the place of the ordinary
eye-piece so that the spectroscopic investigation of
an object can be pursued without any change in the
manner of using the instrument. In a complete
instrument a contrivance is attached to the side by
means of which the substances to be investigated may
be compared with the spectra of known substances:
this apparatus consists of a small stage, a prism for
comparison (38), and a movable scale for measuring
accurately the places of the absorption bands.
Fig. 73 shows a perspective view of the whole
instrument, as fitted to slide into the upper tube of
the microscope in place of the eye-piece; Fig. 74
gives a section showing the internal construction ;
and Fig. 75 gives a section through the plane of the
two screws C and H, exhibiting the slit with its
contrivances for adjustment and the prism for
comparison.
The tube A encloses a second tube carrying a
direct-vision system of five prisms c, and an
achromatic lens l (Fig. 74); by means of a
milled head B, with screw-motion, this inner tube can
be moved up and down, so that the slit situated in
the plane of the screws C and H may be in the focus
of the lens I; consequently the rays from the slit,
after passing through the lens, fall parallel on to
the prisms.
Fig. 74; Section of the
Microspectroscope.
D D is the stage on which the objects for comparison
liquids between plates of glass or in small tubes are
secured within notched edges, by means of metal
springs, which hold the small glasses in such a
position that the light falling on them from the
side, after its passage through the liquid, reaches a
square opening in the middle of the stage, whence, as
Fig. 74 shows, it passes through a side opening o
into the inside of the principal tube, and falls upon
the reflecting prism R, which acts as a prism for
comparison. When the apparatus for comparison is not
required, the square opening in the stage D D is
closed by a sliding plate by means of the screw E, so
that the side-light may be shut out of the
instrument.
Fig. 75: Adjustment for the Slit in the
Microspectroscope.
Fig. 75 gives a section through the plane of the slit
between the screws C and H. The piece n is fixed,
while m is movable, by means of the screw H and an
opposing steel spring, which serves to widen or
narrow the slit. Close over the slit is a covering
plate p, which is moved backward and forward by the
screw C and a spring acting against it, thus enabling
the slit to be lengthened or shortened. The
reflecting prism R covers a part of the slit; if this
slit be open, and the light from the object for
comparison fall from the side at o upon the prism R,
it will be reflected back, and be thrown upon the
system of prisms e, together with the light coming
through the open part of the slit from below (Fig.
74). In this way two spectra are received in
juxtaposition, one produced by the light passing
through the tube G, the other by the light which has
been transmitted through the known liquid upon the
stage D D.
The best contrivance for measuring spectra of absorption
bands is Browning’s Bright-Line Micrometer, shown in Fig. 54. A is a small
mirror by which light from the lamp employed can be reflected through E D
to the lens C, which, by means of a perforated stop, forms a bright pointed
image on the surface of the upper prism, from whence it is reflected to the
eye of the observer. M is a wheel and milled-head. Its rotation carries the
bright point over the spectrum, and the exact amount of motion may be read
off to the 10-1000 on the graduated circle of the wheel. To use this apparatus,
the Fraunhofer lines must be viewed by sending bright daylight through the
spectroscope, and the positions of the principal ones carefully measured,
the reading on the micrometer-wheel being noted down. A Spectrum-map may
then be drawn on cardboard, on a scale of equal parts, and the lines marked
on it, as shown in the upper half of Fig. 55. The lower half of the same
figure shows an absorption-spectrum, with its bands at certain distances
from the Fraunhofer lines. The cardboard Spectrum-map. when once drawn,
should be kept for reference.