143
The somewhat makeshift nature of the bracket, supporting the second plate for third
and larger angular prism, suggests that this arrangement is an afterthought. It implies
a later modification, rather than its original layout as designed or built. The similarity of the two instruments infers that they were developed together. Alas, no photograph of the compound stellar spectroscope was sent to the London exhibition. As
none of this kind of instrument survives, further understanding of its evolution is
unlikely.
8.2.3 Objective Prism Spectroscope
The idea of using an objective prism spectroscope, a prism in front of the objective
lens of a telescope to create a spectrum of a star, was first attempted by Joseph
Fraunhofer (1787–1826) in Munich (DeKosky 2007). The experiment was later
repeated by Johann Lamont (1805–1879) in 1838 using the same equipment
(Hockey et al. 2007: 672; Secchi 1872a: 37). Secchi’s use of the objective prism to
observe stellar spectra is recorded in Le stelle as beginning when he ordered a 6-inch
objective prism from Merz of Munich around 1870. However, writing in 1896, Hale
and Wadsworth suggest that Secchi experimented with such a device as early as
1855, viewing the spectrum of Sirius by attaching a prism to the front objective lens
of a theodolite (Hale and Wadsworth 1896: 54).
The 6-inch objective prism (15.2 cm) survives and images of it were reproduced
in numerous contemporary publications, and it is shown in a photograph supplied to
the 1876 Special Loan Exhibition in London (Science and Art Dept. 1877). The
prism (P) had a refractive angle of 12°, much smaller than the angles of 60° and 37°
for the ones used by Fraunhofer and Lamont. The dispersed starlight was examined
using a cylindrical lens, or an attached eyepiece to widen the stellar spectrum, and
allowed the separation of the D and b lines in Aldebaran (315′) and the splitting of
the D line itself. For optimal operation (minimum deviation), the prism was pivoted
using adjustable screws (a) on the pillars supporting it.
The objective prism was designed for use with either the Cauchoix (15 cm) or the
Merz telescopes (25 cm); when used with the larger instrument, it was attached
using an adaptor plate (Fig. 8.4). The larger dispersion produced by the bigger
instrument (14-feet focal length) produced beautifully resolved spectra for bright
stars. However, for fainter stars the lower dispersion formed by the smaller telescope (7-feet focal length) produced spectra that were brighter, if less resolved. The
extant Merz prism is now fitted with a smaller adaptor plate for use with the
Cauchoix telescope, and this may indicate that it was used more often with the
smaller instrument.
When using an objective prism to make observations of stellar spectra, an inherent difficulty arises in locating the stars to be studied. Earlier observers, such
as Joseph von Fraunhofer (1787–1826), used an offset finder telescope to ameliorate the problem. For his larger Merz refractor, Secchi fitted the object prism with
an auxiliary prism (p) and a viewing hole (c) on opposite edges of the adaptor plate.
8 Angelo Secchi, Instrument Innovator
The somewhat makeshift nature of the bracket, supporting the second plate for third
and larger angular prism, suggests that this arrangement is an afterthought. It implies
a later modification, rather than its original layout as designed or built. The similarity of the two instruments infers that they were developed together. Alas, no photograph of the compound stellar spectroscope was sent to the London exhibition. As
none of this kind of instrument survives, further understanding of its evolution is
unlikely.
8.2.3 Objective Prism Spectroscope
The idea of using an objective prism spectroscope, a prism in front of the objective
lens of a telescope to create a spectrum of a star, was first attempted by Joseph
Fraunhofer (1787–1826) in Munich (DeKosky 2007). The experiment was later
repeated by Johann Lamont (1805–1879) in 1838 using the same equipment
(Hockey et al. 2007: 672; Secchi 1872a: 37). Secchi’s use of the objective prism to
observe stellar spectra is recorded in Le stelle as beginning when he ordered a 6-inch
objective prism from Merz of Munich around 1870. However, writing in 1896, Hale
and Wadsworth suggest that Secchi experimented with such a device as early as
1855, viewing the spectrum of Sirius by attaching a prism to the front objective lens
of a theodolite (Hale and Wadsworth 1896: 54).
The 6-inch objective prism (15.2 cm) survives and images of it were reproduced
in numerous contemporary publications, and it is shown in a photograph supplied to
the 1876 Special Loan Exhibition in London (Science and Art Dept. 1877). The
prism (P) had a refractive angle of 12°, much smaller than the angles of 60° and 37°
for the ones used by Fraunhofer and Lamont. The dispersed starlight was examined
using a cylindrical lens, or an attached eyepiece to widen the stellar spectrum, and
allowed the separation of the D and b lines in Aldebaran (315′) and the splitting of
the D line itself. For optimal operation (minimum deviation), the prism was pivoted
using adjustable screws (a) on the pillars supporting it.
The objective prism was designed for use with either the Cauchoix (15 cm) or the
Merz telescopes (25 cm); when used with the larger instrument, it was attached
using an adaptor plate (Fig. 8.4). The larger dispersion produced by the bigger
instrument (14-feet focal length) produced beautifully resolved spectra for bright
stars. However, for fainter stars the lower dispersion formed by the smaller telescope (7-feet focal length) produced spectra that were brighter, if less resolved. The
extant Merz prism is now fitted with a smaller adaptor plate for use with the
Cauchoix telescope, and this may indicate that it was used more often with the
smaller instrument.
When using an objective prism to make observations of stellar spectra, an inherent difficulty arises in locating the stars to be studied. Earlier observers, such
as Joseph von Fraunhofer (1787–1826), used an offset finder telescope to ameliorate the problem. For his larger Merz refractor, Secchi fitted the object prism with
an auxiliary prism (p) and a viewing hole (c) on opposite edges of the adaptor plate.
8 Angelo Secchi, Instrument Innovator