My inclination has led me, in spite of a lively dread of
incurring a charge of presumption, to address you principally on
that profound and most subtle question, the nature and mode of
formation of the photographic image. I am impelled to do so, not
only because the subject is full of fascination and hopefulness,
but because the wide topics of photographic methods or
photographic applications would be quite unfittingly handled by
the president you have chosen.
I would first direct your attention to Sir James Dewar's
remarkable result that the photographic plate retains
considerable power of forming the latent image at temperatures
approaching the absolute zero--a result which, as I submit,
compels us to regard the fundamental effects progressing in the
film under the stimulus of light undulations as other than those
of a purely chemical nature. But few, if any, instances of
chemical combination or decomposition are known at so low a
temperature. Purely chemical actions cease, indeed, at far higher
temperatures, fluorine being among the few bodies which still
show
[1] Presidential address to the Photographic Convention of the
United Kingdom, July, 1905. _Nature_, Vol. 72, p. 308.
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chemical activity at the comparatively elevated temperature of
-180° C. In short, this result of Sir James Dewar's suggests that
we must seek for the foundations of photographic action in some
physical or intra-atomic effect which, as in the case of
radioactivity or fluorescence, is not restricted to intervals of
temperature over which active molecular vis viva prevails. It
compels us to regard with doubt the role of oxidation or other
chemical action as essential, but rather points to the view that
such effects must be secondary or subsidiary. We feel, in a word,
that we must turn for guidance to some purely photo-physical
effect.
Here, in the first place, we naturally recall the views of Bose.
This physicist would refer the formation of the image to a strain
of the bromide of silver molecule under the electric force in the
light wave, converting it into what might be regarded as an
allotropic modification of the normal bromide which subsequently
responds specially to the attack of the developer. The function
of the sensitiser, according to this view, is to retard the
recovery from strain. Bose obtained many suggestive parallels
between the strain phenomena he was able to observe in silver and
other substances under electromagnetic radiation and the
behaviour of the photographic plate when subjected to
long-continued exposure to light.
This theory, whatever it may have to recommend it, can hardly be
regarded as offering a fundamental explanation. In the first
place, we are left in the dark as to what
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the strain may be. It may mean many and various things. We know
nothing as to the inner mechanism of its effects upon subsequent
chemical actions--or at least we cannot correlate it with what is
known of the physics of chemical activity. Finally, as will be
seen later, it is hardly adequate to account for the varying
degrees of stability which may apparently characterise the latent
image. Still, there is much in Bose's work deserving of careful
consideration. He has by no means exhausted the line of
investigation he has originated.
Another theory has doubtless been in the minds of many. I have
said we must seek guidance in some photo-physical phenomenon.
There is one such which preeminently connects light and chemical
phenomena through the intermediary of the effects of the former
upon a component part of the atom. I refer to the phenomena of
photo-electricity.
It was ascertained by Hertz and his immediate successors that
light has a remarkable power of discharging negative
electrification from the surface of bodies--especially from
certain substances. For long no explanation of the cause of this
appeared. But the electron--the ubiquitous electron--is now known
with considerable certainty to be responsible. The effect of the
electric force in the light wave is to direct or assist the
electrons contained in the substance to escape from the surface
of the body. Each electron carries away a very small charge of
negative electrification. If, then, a body is
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originally charged negatively, it will be gradually discharged by
this convective process. If it is not charged to start with, the
electrons will still be liberated at the surface of the body, and
this will acquire a positive charge. If the body is positively
charged at first, we cannot discharge it by illumination.
It would be superfluous for me to speak here of the nature of
electrons or of the various modes in which their presence may be
detected. Suffice it to say, in further connection with the Hertz
effect, that when projected among gaseous molecules the electron
soon attaches itself to one of these. In other words, it ionises
a molecule of the gas or confers its electric charge upon it. The
gaseous molecule may even be itself disrupted by impact of the
electron, if this is moving fast enough, and left bereft of an
electron.
We must note that such ionisation may be regarded as conferring
potential chemical properties upon the molecules of the gas and
upon the substance whence the electrons are derived. Similar
ionisation under electric forces enters, as we now believe, into
all the chemical effects progressing in the galvanic cell, and,
indeed, generally in ionised solutes.
An experiment will best illustrate the principles I wish to
remind you of. A clean aluminium plate, carefully insulated by a
sulphur support, is faced by a sheet of copper-wire-gauze placed
a couple of centimetres away from it. The gauze is maintained at
a high positive
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potential by this dry pile. A sensitive gold-leaf electroscope is
attached to the aluminium plate, and its image thrown upon the
screen. I now turn the light from this arc lamp upon the wire
gauze, through which it in part passes and shines upon the
aluminium plate. The electroscope at once charges up rapidly.
There is a liberation of negative electrons at the surface of the
aluminium; these, under the attraction of the positive body, are
rapidly removed as ions, and the electroscope charges up
positively.
Again, if I simply electrify negatively this aluminium plate so
that the leaves of the attached electroscope diverge widely, and
now expose it to the rays from the arc lamp, the charge, as you
see, is very rapidly dissipated. With positive electrification of
the aluminium there is no effect attendant on the illumination.
Thus from the work of Hertz and his successors we know that
light, and more particularly what we call actinic light, is an
effective means of setting free electrons from certain
substances. In short, our photographic agent, light, has the
power of expelling from certain substances the electron which is
so potent a factor in most, if not in all, chemical effects. I
have not time here to refer to the work of Elster and Geitel
whereby they have shown that this action is to be traced to the
electric force in the light wave, but must turn to the probable
bearing of this phenomenon on the familiar facts of photography.
I assume that the experiment I have shown you is the most
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fundamental photographic experiment which it is now in our power
to make.
We must first ask from what substances can light liberate
electrons. There are many--metals as well as non-metals and
liquids. It is a very general phenomenon and must operate widely
throughout nature. But what chiefly concerns the present
consideration is the fact that the haloid salts of silver are
vigorously photo-electric, and, it is suggestive, possess,
according to Schmidt, an activity in the descending order
bromide, chloride, iodide. This is, in other words, their order
of activity as ionisers (under the proper conditions) when
exposed to ultra-violet light. Photographers will recognise that
this is also the order of their photographic sensitiveness.
Another class of bodies also concerns our subject: the special
sensitisers used by the photographer to modify the spectral
distribution of sensibility of the haloid salts, _e.g._ eosine,
fuchsine, cyanine. These again are electron-producers under light
stimulus. Now it has been shown by Stoletow, Hallwachs, and
Elster and Geitel that there is an intimate connection between
photo-electric activity and the absorption of light by the
substance, and, indeed, that the particular wave-lengths absorbed
by the substance are those which are effective in liberating the
electrons. Thus we have strong reason for believing that the
vigorous photo-electric activity displayed by the special
sensitisers must be dependent upon their colour absorption. You
will recognise that this is just
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the connection between their photographic effects and their
behaviour towards light.
There is yet another suggestive parallel. I referred to the
observation of Sir James Dewar as to the continued sensitiveness
of the photographic film at the lowest attained extreme of
temperature, and drew the inference that the fundamental
photographic action must be of intra-atomic nature, and not
dependent upon the vis viva of the molecule or atom. In then
seeking the origin of photographic action in photo-electric
phenomena we naturally ask, Are these latter phenomena also
traceable at low temperatures? If they are, we are entitled to
look upon this fact as a qualifying characteristic or as another
link in the chain of evidence connecting photographic with
photo-electric activity.
I have quite recently, with the aid of liquid air supplied to me
from the laboratory of the Royal Dublin Society, tested the
photo-sensibility of aluminium and also of silver bromide down to
temperatures approaching that of the liquid air. The mode of
observation is essentially that of Schmidt--what he terms his
static method. The substance undergoing observation is, however,
contained at the bottom of a thin copper tube, 5 cm. in diameter,
which is immersed to a depth of about 10 cm in liquid air. The
tube is closed above by a paraffin stopper which carries a thin
quartz window as well as the sulphur tubes through which the
connections pass. The air within is very carefully dried by
phosphorus
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pentoxide before the experiment. The arc light is used as source
of illumination. It is found that a vigorous photo-electric
effect continues in the case of the clean aluminium. In the case
of the silver bromide a distinct photo-electric effect is still
observed. I have not had leisure to make, as yet, any trustworthy
estimate of the percentage effect at this temperature in the case
of either substance. Nor have I determined the temperature
accurately. The latter may be taken as roughly about -150° C,
Sir James Dewar's actual measilrements afforded twenty per cent.
of the normal photographic effect at -180° C. and ten per cent.
at the temperature of -252.5° C.
With this much to go upon, and the important additional fact that
the electronic discharge--as from the X-ray tube or from
radium--generates the latent image, I think we are fully entitled
to suggest, as a legitimate lead to experiment, the hypothesis
that the beginnings of photographic action involve an electronic
discharge from the light-sensitive molecule; in other words that
the latent image is built up of ionised atoms or molecules the
result of the photo-electric effect on the illuminated silver
haloid, and it is upon these ionised atoms that the chemical
effects of the developer are subsequently directed. It may be
that the liberated electrons ionise molecules not directly
affected, or it may be that in their liberation they disrupt
complex molecules built up in the ripening of the
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emulsion. With the amount we have to go upon we cannot venture to
particularise. It will be said that such an action must be in
part of the nature of a chemical effect. This must be admitted,
and, in so far as the rearrangement of molecular fabrics is
involved, the result will doubtless be controlled by temperature
conditions. The facts observed by Sir James Dewar support this.
But there is involved a fundamental process--the liberation of the
electron by the electric force in the light wave, which is a
physical effect, and which, upon the hypothesis of its reality as
a factor in forming the latent image, appears to explain
completely the outstanding photographic sensitiveness of the film
at temperatures far below those at which chemical actions in
general cease.
Again, we may assume that the electron--producing power of the
special sensitiser or dye for the particular ray it absorbs is
responsible, or responsible in part, for the special
sensitiveness it confers upon the film. Sir Wm. Abney has shown
that these sensitisers are active even if laid on as a varnish on
the sensitive surface and removed before development. It must be
remembered, however, that at temperatures of about -50° these
sensitisers lose much of their influence on the film; as I have
pointed out in a paper read before the Photographic Convention of
1894.
It. appears to me that on these views the curious phenomenon of
recurrent reversals does not present a problem hopeless of
explanation. The process of photo-
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ionisation constituting the latent image, where the ion is
probably not immediately neutralised by chemical combination,
presents features akin to the charging of a capacity--say a Leyden
jar. There may be a rising potential between the groups of ions
until ultimately a point is attained when there is a spontaneous
neutralisation. I may observe that the phenomena of reversal
appear to indicate that the change in the silver bromide
molecule, whatever be its nature, is one of gradually increasing
intensity, and finally attains a maximum when a return to the
original condition occurs. The maximum is the point of most
intense developable image. It is probable that the sensitiser--in
this case the gelatin in which the bromide of silver is
immersed--plays a part in the conditions of stability which are
involved.
Of great interest in all our considerations and theories is the
recent work of Wood on photographic reversal. The result of this
work is--as I take it--to show that the stability of the latent
image may be very various according to the mode of its formation.
Thus it appears that the sort of latent effect which is produced
by pressure or friction is the least stable of any. This may be
reversed or wiped out by the application of any other known form
of photographic stimulus. Thus an exposure to X-rays will
obliterate it, or a very brief exposure to light. The latent
image arising from X-rays is next in order of increasing
stability. Light action will remove this. Third in order is a
very brief light-shock or sudden flash. This
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cannot be reversed by any of the foregoing modes of stimulation,
but a long-continued undulatory stimulus, as from lamp-light,
will reverse it. Last and most stable of all is the gradually
built-up configuration due to long-continued light exposure. This
can only be reversed by overdoing it according to the known facts
of recurrent reversal. Wood takes occasion to remark that these
phenomena are in bad agreement with the strain theory of Bose. We
have, in fact, but the one resource--the allotropic modification
of the haloid--whereby to explain all these orders of stability.
It appears to me that the elasticity of the electronic theory is
greater. The state of the ionised system may be very various
according as it arises from continued rhythmic effects or from
unorganised shocks. The ionisation due to X-rays or to friction
will probably be quite unorganised, that due to light more or
less stable according to the gradual and gentle nature of the
forces at work. I think we are entitled to conclude that on the
whole there is nothing in Wood's beautiful experiments opposed to
the photo-electric origin of photographic effects, but that they
rather fall in with what might be anticipated according to that
theory.
When we look for further support to the views I have laid before
you we are confronted with many difficulties. I have not as yet
detected any electronic discharge from the film under light
stimulus. This may be due to my defective experiments, or to a
fact noted by Elster and Geitel concerning the photo-electric
properties of gelatin.
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They obtained a vigorous effect from Balmain's luminous paint,
but when this was mixed in gelatin there was no external effect.
Schmidt's results as to the continuance of photo-electric
activity when bodies in general are dissolved in each other lead
us to believe that an actual conservative property of the medium
and not an effect of this on the luminous paint is here involved.
This conservative effect of the gelatin may be concerned with its
efficacy as a sensitiser.
In the views I have laid before you I have endeavoured to show
that the recent addition to our knowledge of the electron as an
entity taking part in many physical and chemical effects should
be kept in sight in seeking an explanation of the mode of origin
of the latent image.[1]
[1] For a more detailed account of the subject, and some
ingenious extensions of the views expressed above, see
_Photo-Electricity_, by H. Stanley Allen: Longmans, Green & Ca.,
1913.
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