Dr. J. G. Hofmann,
originally of German extraction, established a business
in Paris as an optical and philosophical instrument
maker. The firm was active between the years
1850-1875 trading at rue de Bucy. Later (1863-1875),
the firm was referred to as l' Institute d'
Optique Paris.
This type of direct
vision spectroscope was first introduced by
Hofmann in 1862. It utilizes a series of five prisms
cemented together consisting of two different types of
glass (crown and flint) having differing indices of
refraction. With the example shown here, the spectrum
is focused by rack and pinion and there is a mechanism
that alters the angle of the telescope relative to the
prisms which allows it to focus on different parts of
the spectrum. There is an accessory telescope at a near
right angle to the main tube that projects a wave
length scale onto the spectrum. In addition, there is a
mechanism in front of the slits that allows the use of
a small comparison prism. Later, other manufacturers
copied this Hofmann design. One such instrument by
Franz Schmidt & Haensch, Berlin is represented
in this collection.
The following was
extracted from the Reports of the United States
Commissioners to the Paris Universal Exposition,
1867, Vol. 3
The most convenient form
of spectroscope for ordinary uses is the direct-vision
spectroscope, in which the dispersion is effected by
prisms contained within the tube of the observing
telescope itself, as in the case of Amici's pocket
spectroscope mentioned above. Tbe Abbe Moigno, in his
journal Les Mondes, states that Jansen was the first to
design a spectroscope in this form, which seems not to
have essentially differed in principle from that of
Amici, but which was considerably more powerful. In the
instrument of Amici the ray, after having been
dispersed by one prism, is brought by reflection into
its original direction, the dispersion remaining. In
Jansen's, a second pair of prisms is placed immediately
behind the first, which is in all respects similar to
that. The effect is therefore to double the
dispersion.
A form of the instrument
superior to either of these has been contrived by
Hoffmann, of Paris, the very able constructor to whose
skill the investigators of the higher optics have been
so much indebted, and who has furnished to Father
Secchi and to Mr. Huggins the instruments which have
enabled them to prosecute so successfully the spectral
analysis of stellar and nebular light. In Mr.
Hoffmann's direct-vision spectroscope, the apparatus
for dispersion consists of five prisms, three of crown
glass and two of flint glass, cemented together into
one system with their refracting angles alternately in
opposite directions. Tbe arrangement resembles that of
the group of letters AVAVA, in which the cross-line of
the letters A indicates the path of the light through
the system. The dispersion is differential, the angles
of the prisms being so chosen as to compensate the mean
refraction; and the mean ray emerges parallel to the
direction of incidence. But as the extreme rays of the
spectrum produced by the dispersion are necessarily not
parallel to the same direction, the tube is jointed at
a point just behind the system of prisms, and the part
near the eye has a liberty of lateral motion sufficient
to enable the observer to bring any portion of the
spectrum into the field of vision. The angles actually
given to the several prisms at their summits are ninety
degrees for the two flint-glass prisms, represented in
the group of letters above by the two Vs, and also for
the central prism of crown glass. The angles of the
extreme crownglass prisms are only sixty-nine
degrees.
Spectrum of a compact
fluorescent lamp as seen through the
spectroscope
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Instruments