149
8.5 Secchi Disc
Although Secchi’s name is now associated with the tool still used to measure water
transparency, he was not the first to use such a device (Lorenz 1863: 178–85).
However, his became better known than earlier researchers in the field when in 1865
he published his results and the systematic method by which he made his measurements (Cialdi and Secchi 1865: 100–4). As knowledge of the Secchi method spread,
his association with the disc used to make the measurement was assured. Secchi’s
description of the size of the discs employed, the depth of deployment and the
recording of solar altitude guaranteed its adoption as the standard process by which
water transparency measurements were made. Joseph R. Lorenz (1825–1911),
whose similar work predated Secchi’s research by 7 years, commented in 1898 that
he preferred the technique to be called the “Disc-System” (Lorenz 1898: 69).
History judges which name prevails.
Secchi undertook his transparency measurements using the papal corvette,
“Immacolata Concezione” with Captain Alessandro Cialdi (1807–1882), in the
Tyrrhenian Sea offshore from Civitavecchia to Terracina near Rome (Altamore and
Maffeo 2012: 243–8). The modern Secchi disc, 20 cm (8 inches) in diameter, with
a checkerboard pattern, was derived from two circles measuring 43 cm and 237 cm
made of white painted canvas used by Secchi and Cialdi. Initially described in
Comptus Rendus in 1864, later works by the same authors in the journal Il Nuovo
Cimento (Secchi 1864) and a book (Secchi 1866) provided a schematic explanation
of their methods that was readily accessible to the reader and hence ensured the
popularity of the technique (Zielinski, Chap. 14 in this volume).
8.6 Concluding Remarks
Secchi’s achievements in relation to the scientific instruments that he used, developed or designed indicate his broad skill base and interests across the entire range
of the physical sciences. His extensive knowledge of astrophysics, meteorology,
geodesy and the electrical sciences allowed him to investigate especially practical
aspects of these disciplines.
He was keen to exchange ideas with instrument makers to modify and improve
designs as has been shown by his discourse with the Merzs (Chinnici 2017a, b,
39–68). He also realised the need for a research team, appointing several assistants
to ensure observational continuity and that talented observers were used. One of
these, father Enrico Cappelletti (1831–1899), is probably responsible for the faithfully drawings of planetary observations made at the Observatory (Chinnici 2019,
70; McKim and Sheehan, Chap. 6 in this volume).
While open to discuss his ideas and observations with colleagues, Secchi exhibited traits of academic suspicion when he barred Jules Janssen from the Romano
Collegio Observatory after his initial visit in 1862 (Chinnici 2019: 160–1). As a
consequence of unfounded suspicions of plagiarism, his works were never translated into English (Chinnici 2019: 292–296).
8 Angelo Secchi, Instrument Innovator
8.5 Secchi Disc
Although Secchi’s name is now associated with the tool still used to measure water
transparency, he was not the first to use such a device (Lorenz 1863: 178–85).
However, his became better known than earlier researchers in the field when in 1865
he published his results and the systematic method by which he made his measurements (Cialdi and Secchi 1865: 100–4). As knowledge of the Secchi method spread,
his association with the disc used to make the measurement was assured. Secchi’s
description of the size of the discs employed, the depth of deployment and the
recording of solar altitude guaranteed its adoption as the standard process by which
water transparency measurements were made. Joseph R. Lorenz (1825–1911),
whose similar work predated Secchi’s research by 7 years, commented in 1898 that
he preferred the technique to be called the “Disc-System” (Lorenz 1898: 69).
History judges which name prevails.
Secchi undertook his transparency measurements using the papal corvette,
“Immacolata Concezione” with Captain Alessandro Cialdi (1807–1882), in the
Tyrrhenian Sea offshore from Civitavecchia to Terracina near Rome (Altamore and
Maffeo 2012: 243–8). The modern Secchi disc, 20 cm (8 inches) in diameter, with
a checkerboard pattern, was derived from two circles measuring 43 cm and 237 cm
made of white painted canvas used by Secchi and Cialdi. Initially described in
Comptus Rendus in 1864, later works by the same authors in the journal Il Nuovo
Cimento (Secchi 1864) and a book (Secchi 1866) provided a schematic explanation
of their methods that was readily accessible to the reader and hence ensured the
popularity of the technique (Zielinski, Chap. 14 in this volume).
8.6 Concluding Remarks
Secchi’s achievements in relation to the scientific instruments that he used, developed or designed indicate his broad skill base and interests across the entire range
of the physical sciences. His extensive knowledge of astrophysics, meteorology,
geodesy and the electrical sciences allowed him to investigate especially practical
aspects of these disciplines.
He was keen to exchange ideas with instrument makers to modify and improve
designs as has been shown by his discourse with the Merzs (Chinnici 2017a, b,
39–68). He also realised the need for a research team, appointing several assistants
to ensure observational continuity and that talented observers were used. One of
these, father Enrico Cappelletti (1831–1899), is probably responsible for the faithfully drawings of planetary observations made at the Observatory (Chinnici 2019,
70; McKim and Sheehan, Chap. 6 in this volume).
While open to discuss his ideas and observations with colleagues, Secchi exhibited traits of academic suspicion when he barred Jules Janssen from the Romano
Collegio Observatory after his initial visit in 1862 (Chinnici 2019: 160–1). As a
consequence of unfounded suspicions of plagiarism, his works were never translated into English (Chinnici 2019: 292–296).
8 Angelo Secchi, Instrument Innovator