828
Laroche opened a studio at 65 Oxford Street, London,
before 1848—under the name ‘Silvester Laroche’—ini-
tially making daguerreotypes, examples of which were
shown at the Great Exhibition in 1851. He later used the
wet collodion process. The studio continued to operate
until at least 1862.
In the 1854 court case, Talbot was represented by,
amongst others, the eminent scientist William Grove
FRS, while Laroche’s solicitor was Peter Wickens Fry
who had worked with Archer in the development of
the collodion method. The verdict confi rmed Talbot’s
pre-eminence as the inventor of negative/positive pho-
tography, but also confi rmed that collodion was not
covered by his patents.
John Hannavy
LATENT IMAGE
A latent image can be most simply defi ned as a hidden
or invisible image formed by brief exposure to light,
which can be revealed to the naked eyed only by the
chemical action of a developing agent.
The fi rst steps towards the concept of a photographic
latent image are often traced back to W.H.F. Talbot’s
work in September 1840 that led to the introduction of
his Calotype process. However, there can be little doubt
that L.J.M. Daguerre, albeit imperfectly, recognised
the presence of some invisible pre-image state initiated
by light, perhaps as early as 1837. His daguerreotype
process required the use of a silvered copper plate made
light sensitive by exposure to iodine vapour. Little or
no visible image was produced after several minutes of
exposure in the camera, but it was found that a distinct
image appeared after treatment (development) with
heated mercury vapour. Daguerre is reputed to have dis-
covered his developing agent purely by accident when
he left a collection of old plates in a cupboard containing
an uncovered basin fi lled with liquid mercury. The term
“latent image” was certainly used in 1839 in connec-
tion with the daguerreotype process. An account of the
offi cial report of Daguerre’s technique published in Le
Constitutionnel, 21 August 1839 states; “After 4 to 10
minutes, according to the period of the day, according
to the season, and to the intensity of light, the image of
immobile objects from which the lens receives the light,
becomes perfectly imprinted on the plate, although this
image is yet invisible and only latent [“seulement lat-
ente”].... But this image, that is yet, so to say, in a state
of an unformed chrysalis, what consequently comes to
reveal it out of its swaddling clothes? It is the vapour of
mercury, from mercury heated to 60 Reaumur.”
Talbot’s concept of the latent image has been more
widely recognized, perhaps because of his use of
techniques closer to modern practice. The story is well
documented. In his original process announced in1839,
photogenic drawing, paper coated with silver halide salts
was exposed in a camera until an image appeared. The
process was very slow. Many minutes, even hours, were
required to produce a satisfactory picture. In the late
summer of 1840 he recommenced some earlier experi-
ments he had begun using gallic acid. His notebook of
September 21 refers to “an exciting liquid” a mixture
of silver nitrate, acetic acid and gallic acid. His entry
of September 23 records “The same exciting liquid
was diluted with an equal bulk of water, and some very
remarkable effects were obtained. Half a minute suffi ces
for the Camera the paper when removed is often blank
but when kept in the dark the picture begins to appear
spontaneously, and keeps improving for several minutes.
.. .” On the same page in the context of reviving what
he called old or faded pictures, he refers to a “kind of
latent picture....” Talbot had discovered the latent im-
age, which could be revealed using a developer, in his
case, gallic acid. He later wrote in a letter to the Liter-
ary Gazette, 19 February, 1841, “I know few things in
the range of science more surprising than the gradual
appearance of the picture on a blank sheet, especially
the fi rst time the experiment is witnessed.”
The mechanism of latent image formation intrigued
and baffl ed the pioneers and continued to be the subject
of speculation and dispute throughout the nineteenth
century. According to the popular 1850s guide, A
Manual of Photographic Chemistry, by T. Frederick
Hardwich (third edition, 1856), “the ray of light deter-
mines a molecular change of some kind in the particles
of Iodide of Silver forming the sensitive surface.” The
American authority, M.Carey Lea, offered a different
explanation. In The Photographic News, (11 August
1865) he claimed “...the production of a developable
image in the camera upon an iodo-bromised fi lm is a
purely physical phenomenon, that no decomposition
of the silver salts takes place, no separation of iodine.
By the end of the nineteenth century, most latent image
formation theorists were sharply divided in favour of
one or the other of the two approaches cited above, the
chemical or the physical. The former group suggested
the formation of an unknown sub halide salt while those
favouring the latter argued that the action of light caused
some change in the physical character of the silver salts.
It was not until photography became a giant industry
supported by systematic scientifi c research facilities
in the 20th century that the mysteries of latent image
formation began to be unravelled.
Modern scientifi c explanations are based on the
concentration speck theory of R.S. Sheppard and col-
leagues working at the Kodak Research Laboratories
in the 1920s and the mechanism proposed in 1938
by Professors R.W. Gurney and N.F. Mott based at
Bristol University. The detailed theories are complex
and the supporting experimental evidence requires an