145
making observations with the objective prism. In operation, an individual spectrum
was selected using a sliding plate with a screw adjustment that carried a positive
eyepiece, or cylindrical lens, if more light was needed for faint stars. Alas, the
instrument no longer exists so it is not clear whether other examples were made.
With the introduction of more sensitive photographic dry film emulsions in the late
1870s, one no longer needed to observe with the naked eye. Because multiple star
spectra could be recorded on each photographic plate, locating the stars became less
critical.
8.2.4 Grating Spectroscope
Although it is recorded that Secchi used a grating spectroscope—Lewis
M. Rutherfurd (1816–1892) supplied him with a grating in 1875—it is unlikely that
it was successfully used for stellar observations (Hearnshaw 1986: 10; Baum
2007b). The instrument is not described in Le stelle, but two photographs showing
the spectroscope attached to the Merz telescope were exhibited at the 1876 Special
Loan Exhibition in London (Science and Art Dept. 1877). Secchi, commenting on
the instrument, said: “The spectra formed thus by interference possess a clarity and
certainty which by far surpass those from prisms” (Secchi 1877b: 27). It is almost
certain that Secchi is describing an observation of the solar spectrum using the setup
shown in the 1876 photograph of the Merz telescope at the Collegio Romano
Observatory. The gratings, manufactured by Rutherfurd using his diamond ruling
engine, were far too inefficient to produce stellar spectra visible to the naked eye,
due to the low reflectivity of the speculum metal substrate. Hearnshaw comments,
“their use [gratings] for stellar work at this time was very limited, due their inefficiency which resulted from the available light being divided between several diffraction orders” (Hearnshaw 1986: 10).
8.3 Thermoheliometer (Actinometer)
Secchi’s attempt to measure the temperature of the Sun is less well known than his
exploits in the field of spectroscopy and in attempting to measure the solar constant;
in this, he was following in the footsteps of earlier researchers. Secchi was motivated during his American exile after being shown the work of Joseph Henry
(1797–1878), who used a thermopile to measure the relative temperature of sunspots. After Secchi extended Henry’s work, published in 1851, a contemporary
account suggests that Secchi gave Henry too little credit for the methods he copied
(Secchi 1851; Mayer 1880; 503). Such controversy was to plague Secchi throughout his career (Chinnici 2019: 259–305).
The instrument he developed was based on one conceived by the Scottish physicist, John James Waterson (1811–1883), and was used in India to measure solar
8 Angelo Secchi, Instrument Innovator
making observations with the objective prism. In operation, an individual spectrum
was selected using a sliding plate with a screw adjustment that carried a positive
eyepiece, or cylindrical lens, if more light was needed for faint stars. Alas, the
instrument no longer exists so it is not clear whether other examples were made.
With the introduction of more sensitive photographic dry film emulsions in the late
1870s, one no longer needed to observe with the naked eye. Because multiple star
spectra could be recorded on each photographic plate, locating the stars became less
critical.
8.2.4 Grating Spectroscope
Although it is recorded that Secchi used a grating spectroscope—Lewis
M. Rutherfurd (1816–1892) supplied him with a grating in 1875—it is unlikely that
it was successfully used for stellar observations (Hearnshaw 1986: 10; Baum
2007b). The instrument is not described in Le stelle, but two photographs showing
the spectroscope attached to the Merz telescope were exhibited at the 1876 Special
Loan Exhibition in London (Science and Art Dept. 1877). Secchi, commenting on
the instrument, said: “The spectra formed thus by interference possess a clarity and
certainty which by far surpass those from prisms” (Secchi 1877b: 27). It is almost
certain that Secchi is describing an observation of the solar spectrum using the setup
shown in the 1876 photograph of the Merz telescope at the Collegio Romano
Observatory. The gratings, manufactured by Rutherfurd using his diamond ruling
engine, were far too inefficient to produce stellar spectra visible to the naked eye,
due to the low reflectivity of the speculum metal substrate. Hearnshaw comments,
“their use [gratings] for stellar work at this time was very limited, due their inefficiency which resulted from the available light being divided between several diffraction orders” (Hearnshaw 1986: 10).
8.3 Thermoheliometer (Actinometer)
Secchi’s attempt to measure the temperature of the Sun is less well known than his
exploits in the field of spectroscopy and in attempting to measure the solar constant;
in this, he was following in the footsteps of earlier researchers. Secchi was motivated during his American exile after being shown the work of Joseph Henry
(1797–1878), who used a thermopile to measure the relative temperature of sunspots. After Secchi extended Henry’s work, published in 1851, a contemporary
account suggests that Secchi gave Henry too little credit for the methods he copied
(Secchi 1851; Mayer 1880; 503). Such controversy was to plague Secchi throughout his career (Chinnici 2019: 259–305).
The instrument he developed was based on one conceived by the Scottish physicist, John James Waterson (1811–1883), and was used in India to measure solar
8 Angelo Secchi, Instrument Innovator