132
color photography experiments were also based on
Sir John Herschel’s systematic investigation of solar
spectra recorded in silver-chloride impregnated paper.
This type of photographic recording technique is often
referred to as heliochromy (sun-coloring) and the images
as heliochromes.
Instead of using paper as the material substrate,
Becquerel coated a silver subchloride (Ag 2 Cl) emulsion
on a polished silver plate. Actually, Becquerel preferred
chlorination by a galvanic process. The silver plate was
immersed as the positive pole in a weak (1:8) hydro-
chloric acid, the negative pole being a platinum sheet.
Within a few minutes the process was completed. By
timing the process, he could accurately control the thick-
ness of the layer. The best colors were obtained when
the galvanic process was stopped at the “fourth-order
violet” which corresponded to a thickness of 1/588 mm.
Employing such plates, the colors of the recorded solar
spectrum were much brighter than previously recorded
spectra. However, the same problem of fi xing the images
remained unsolved, and despite trying hard, Becquerel
could not fi nd a solution. The images faded rapidly under
daylight illumination. They had to be stored in complete
darkness. In addition to solar spectra recorded in 1848,
Becquerel succeeded also in recording color photo-
graphs of objects. He recorded some colored engravings
and brightly dressed dolls which all required between
ten and twelve hours exposure in bright sunlight. For
example, at the 1855 Exposition Universelle in Paris,
such color photographs were on display in an almost
dark tent to preserve the photographs as long as possible.
The only light inside was a single candlelight.
Lord Rayleigh (1842–1919) suggested in 1887 that
the images obtained in the Becquerel experiment could
be explained in this way: “The various parts of the fi lm
of subchloride of silver with which the metal is coated
may be conceived to be subjected, during exposure,
to stationary luminous waves of nearly defi nite wave-
length, the effect of which might be to impress upon
the substance a periodic structure recurring at intervals
equal to half the wave-length of the light.” Then the
recording technique is: “to produce just such a modifi ca-
tion of the fi lm as would cause it to refl ect most copi-
ously that particular kind of light.” In 1889 Otto Wiener
(1862–1927) investigated and explained the experiments
by Seebeck and Becquerel as well as interferential color
photography invented in 1891 by Gabriel Lippmann
(1845–1921). The colors observed in the experiments
are obtained in two ways: caused by interference or by
absorption and bleach-out process. Becquerel’s process
may, to some extent, be based on interference, but is
much more similar to color recordings on chloride paper.
In this case a chemical bleaching process of pigments
is behind the colors. The light-sensitive substances are
bleached out only by those kinds of light which they
absorb, while they are not destroyed by light of their own
color. One of Becquerel’s solar spectra recorded in 1848
has been preserved and belongs to the collection of the
Musée national des techniques (Conservatoire national
des arts and métiers) in Paris. It is stored in complete
darkness in a box. It is very diffi cult to get permission
to view the photograph. Only on a few occasions have
researchers been able to view and investigate the re-
corded spectrum. However, ordinary photographic color
prints exist of Becquerel’s photograph; for example, a
photo is reproduced on page 29 in Histoire mondiale de
la photographie en colours, a book by Roger Bellone
and Luc Fellot published in 1981.
Hans I. BjelkhagenBiography
Alexandre Edmond Becquerel was born in 1820, the
son of Antoine César Becquerel who was a pioneer
in electrochemical science. Alexandre Edmond’s son
Antoine Henri was the famous scientist who discovered
radioactivity in 1896 for which he shared the Nobel Prize
in Physics in 1903. Alexandre Edmond is known for his
studies in light, photochemistry, and phosphorescence.
For example, he is the inventor of the phosphoroscope.
Alexandre Edmond discovered the photovoltaic effect in- Doctor of Science in 1840 and professor of Phys-
ics at the Conservatoire national des arts and métiers
(National Academy of Arts and Trade) in 1852. When
his father died in 1878, he succeeded him as a professor
at the Muséum d’Histoire naturelle (Museum of Natural
History) in Paris. He is the author of the book: La Lu-
mière ses causes et ses effets, published in 1868. In the
fi eld of photography he is known mainly for his work in
color recording experiments. He died in 1891.
See Also: Herschel, Sir John Frederick William; and
Lippmann, Gabriel.
Further Reading
Becquerel, Edmond, De l’image photographique colorée du
spectre solaire. Comptes Rendus Hebdomadaires des Séances
de l’Académie des Sciences 26 (1848), 181–183.
Becquerel, Edmond, De l’image photochromatique du spectre
solaire, et des images colorées obtenues dans la chambre
obscure. Comptes Rendus Hebdomadaires des Séances de
l’Académie des Sciences 27 (1848), 483.
Becquerel, Edmond, De l’image photographique colorée du
spectre solaire. Annales de Chemie et de Physique 22 (1848),
451–459.
Becquerel, Edmond, De l’image photochromatique du spectre
solaire et des images colorées obtenues à la chambre obscure.
Annales de Chemie et de Physique 25 (1849), 447–474.
Becquerel, Edmond, Nouvelles recherches sur les impressions
colorées produites lors de l’action chimique de la lumière.
Annales de Chemie et de Physique 42 (1854), 81–106.
Becquerel, Edmond, La lumière ses causes et ses effets (F. Didot
Frères, Fils et CIE, Paris, 1868).