140
Those with direct vision, built with strong prisms from Hofman (sic) and Merz, were found
very convenient and comfortable to use. Mr. Huggins has preferred the common form of
angular prisms such as in chemistry, adapting the dimensions to the eyepieces. We have
used both systems without finding much of a preference for one over the other, because
whatever light is lost in the former by absorption is not much different from what you lose
in the two oblique reflections of the latter. It is true, however, that it is easier to have a good
spectroscope with angular prisms that one with direct vision. (Secchi 1877a: 39)
3
Secchi then describes further the disadvantages of the direct-vision spectroscope
when used for stellar observation:
Direct-vision spectroscopes are more comfortable, but they lengthen the instrument a lot,
and in some cases they are unusable due to the space constraints of the observatory dome.
Moreover, an angular prism can easily be removed, thus diminishing the dispersion and so
adaptable to dimmer objects, which cannot be done with the direct-vision spectroscope.
(Secchi 1877a: 39)
4
Considering the strengths and weakness of the respective instruments, it is not
surprising that astronomers soon combined both designs. In all likelihood several
people probably conceived of the idea around the same time (Fig. 8.2). It is not clear
who first applied this layout to the spectroscope, but an early example is described
by William Huggins in 1868:
… I had in my possession two very fine direct-vision prisms on AMICI’S principle, which
had been made for me by HOFMANN of Paris, induced me to attempt to combine in one
instrument several simple prisms with one or two compound prisms which give direct
vision. An instrument constructed in this way, as will be seen from the following description, possesses several not unimportant advantages*. (Huggins 1868: 536)
The footnote (*) attached to this statement reads: “An apparatus in many respects
superior to the one here described has been constructed since–October 1868”. It is
interesting to speculate as to the maker’s identity of the superior instrument, and it
is quite possible that Secchi may have had the same idea as Huggins. Whatever the
course of events, Secchi describes and illustrates an advanced form of the compound spectroscope in Le stelle.
In the compound instrument illustrated in Fig. 8.3, the adjustable slit with collimating and cylindrical lenses is mounted in the tube (UKB) along with bracket P
that carries an ebonite mounted clamp used to generate sparks for comparison
spectra. Attached to the aforementioned tube, at the bottom, is a plate (XYZ) for two
angular prisms with automatic adjustment for minimum deviation. Light from the
3 Quelli a visione diretta costruiti con forti prismi da Hofman e da Merz, furono trovati molto utili
e di comodo servizio. Il signor Huggins preferì la forma comune a prismi angolari come quelli di
chimica, adattandone le dimensioni agli oculari. Noi abbiamo usato di ambedue i sistemi senza
trovarci una preponderanza considerabile per questi ultimi; giacchè quello che nei primi si perde
per assorbimento è compensato dagli altri, per ciò che perdesi in riflessioni troppo oblique. È vero
però che è più facile avere un buono spettroscopio a prismi angolari che uno a visione diretta.
4 Gli spettroscopii a visione diretta sono più comodi, ma allungando molto lo strumento, in certi
casi, per la strettezza delle cupole mobili possono divenire inservibili. Di più si può facilmente
togliere un prisma agli angolari e diminuirne la dispersione il che non può farsi con i diretti, e così
adattarli agli oggetti i più deboli.
K. L. Johnson
Those with direct vision, built with strong prisms from Hofman (sic) and Merz, were found
very convenient and comfortable to use. Mr. Huggins has preferred the common form of
angular prisms such as in chemistry, adapting the dimensions to the eyepieces. We have
used both systems without finding much of a preference for one over the other, because
whatever light is lost in the former by absorption is not much different from what you lose
in the two oblique reflections of the latter. It is true, however, that it is easier to have a good
spectroscope with angular prisms that one with direct vision. (Secchi 1877a: 39)
3
Secchi then describes further the disadvantages of the direct-vision spectroscope
when used for stellar observation:
Direct-vision spectroscopes are more comfortable, but they lengthen the instrument a lot,
and in some cases they are unusable due to the space constraints of the observatory dome.
Moreover, an angular prism can easily be removed, thus diminishing the dispersion and so
adaptable to dimmer objects, which cannot be done with the direct-vision spectroscope.
(Secchi 1877a: 39)
4
Considering the strengths and weakness of the respective instruments, it is not
surprising that astronomers soon combined both designs. In all likelihood several
people probably conceived of the idea around the same time (Fig. 8.2). It is not clear
who first applied this layout to the spectroscope, but an early example is described
by William Huggins in 1868:
… I had in my possession two very fine direct-vision prisms on AMICI’S principle, which
had been made for me by HOFMANN of Paris, induced me to attempt to combine in one
instrument several simple prisms with one or two compound prisms which give direct
vision. An instrument constructed in this way, as will be seen from the following description, possesses several not unimportant advantages*. (Huggins 1868: 536)
The footnote (*) attached to this statement reads: “An apparatus in many respects
superior to the one here described has been constructed since–October 1868”. It is
interesting to speculate as to the maker’s identity of the superior instrument, and it
is quite possible that Secchi may have had the same idea as Huggins. Whatever the
course of events, Secchi describes and illustrates an advanced form of the compound spectroscope in Le stelle.
In the compound instrument illustrated in Fig. 8.3, the adjustable slit with collimating and cylindrical lenses is mounted in the tube (UKB) along with bracket P
that carries an ebonite mounted clamp used to generate sparks for comparison
spectra. Attached to the aforementioned tube, at the bottom, is a plate (XYZ) for two
angular prisms with automatic adjustment for minimum deviation. Light from the
3 Quelli a visione diretta costruiti con forti prismi da Hofman e da Merz, furono trovati molto utili
e di comodo servizio. Il signor Huggins preferì la forma comune a prismi angolari come quelli di
chimica, adattandone le dimensioni agli oculari. Noi abbiamo usato di ambedue i sistemi senza
trovarci una preponderanza considerabile per questi ultimi; giacchè quello che nei primi si perde
per assorbimento è compensato dagli altri, per ciò che perdesi in riflessioni troppo oblique. È vero
però che è più facile avere un buono spettroscopio a prismi angolari che uno a visione diretta.
4 Gli spettroscopii a visione diretta sono più comodi, ma allungando molto lo strumento, in certi
casi, per la strettezza delle cupole mobili possono divenire inservibili. Di più si può facilmente
togliere un prisma agli angolari e diminuirne la dispersione il che non può farsi con i diretti, e così
adattarli agli oggetti i più deboli.
K. L. Johnson