308 



NA rURE 



[July 27, 1905 



remarks upon the subject or upon other phenomena noted 

 at the time of the eclipse. A. Lawrence Rotch. 



. Blue Hill Observatory, Hyde Park, Mass., U.S.A. 



The observations may be summarised as follows : — 

 Observations of Shadow Bands, August 30, 1905. 



Place 



(Situation and altitude) 



Observers 



1. Direction of bands 



2. Direction of motion 



3. Velocity, 



4. Width of bands, 



5. Distance apart, 



Remarks : 



Direction of the wind before totality after 



totality and direction from which upper 



clouds (if any) came 



THE LATENT IMAGE. 



W 



\Y 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 con- 

 siderable power of forming the latent image at tempera- 

 tures 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 un- 

 dulations 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 

 chemical activity at the comparatively elevated tempera- 

 ture of — 180° C. In short, this result of Si* James Dewar's 

 suggests that we must seek for the foundations of photo- 

 graphic action in some physical or intra-atomic effect 

 which, as in the case of radio-activity 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 o.xidation 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 

 Mr. 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 speci- 

 ally 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 radia- 

 tion 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 explana- 

 tion. In the first place, we are left in the dark as to what 

 the strain may be. It may mean many and various things. 

 We know nothing as to the inner mechanism of its effects 



of the United KinHdom, 1905. 



1 Address to the Photographic Co 

 By J. Joly, F.R.S. 



NO. 1865, VOL. 72] 



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 Mr. Bose's work deserving of careful considera- 

 tion. He has by no means exhausted the line of investi- 

 gation 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 con- 

 nects light and chemical phenomena through the inter- 

 mediary 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 re- 

 sponsible. The effect of the electric force in the light wave 

 is to direct or assist the electrons contained in the sub- 

 stance to escape from the surface of the body. Each 

 electron carries away a very small charge of negative 

 electrification. If, then, a body is originally charged nega- 

 tively, 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 mole- 

 cules 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, gene- 

 rally in ionised solutants. 



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 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. 



.\gain, 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 generally what we call actinic 

 light, is an effective means of freeing the electron from 

 certain substances. In short, our photographic agent, 

 light, has the power of evoking 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 

 fundamental photographic experiment which it is now in 

 our power to make. 



