Volltext Seite (XML)
652 THE PHOTOGRAPHIC NEWS. [OcTOBER 9, 1885. Is it true, then, that it is reduced to a sub-bromide? I think this is untenable. There is not such a thing as two chemical substances having exactly the same outward properties, and no difference has yet been shown to exist in the properties of the parts of the plate acted on by light, as compared with the other portions. The colour is the same. When placed in a material such as ammonia or hyposulphite of soda, the exposed and un- exposed portions of the film are dissolved off with equal rapidity. In short, we must believe that in the film there are two chemical substances of distinctly different composition—a normal bromide and a sub-bromide—having identical colours and solubility, before we can accept the theory. It is hard to do this if a better theory is at hand. But, it may he argued, it is not necessary to believe that the whole of the portion acted on by light is reduced to a sub-bromide. If only the most minute portion is so reduced, the rest of the action is carried on catalytically. In some such way it may seem possible to bolster up a theory which is at best weak and clumsy. There is another, and to my mind an infinitely superior theory, which we may designate the dynamic theory, and this we may consider for a few minutes. As we all know, chemical substances are made up of very small particles or atoms. These particles or atoms are in a continual state of motion—of vibration—and are subject to the influence of the various forms of force, amongst which is light. When placed under suitable conditions, many chemical substances, including silver salts, are altered or changed by the action of light; in ordinary phraseology, they are “ sensitive.” Besides the silver salts, one of the most common instances of sensitiveness is to be found in the case of certain calcium salts, which become luminous on exposure to light. I have here a plate coated with a sensitive substance. A small experiment with it may help me to explain matters. [The lecturer here exposed a plate covered with Balmain’s luminous paint, below a negative, to the action of magnesium light, and exhibited the luminous picture on the room being darkened.] Now here is a case of the action of light which may help to explain the disturbance due to light on a photographic plate. In the first place, let us ask ourselves the question : Is the material of which this paint is composed destroyed or decom posed by the action of light ? No. It remains exactly as it was, chemically considered. Then what is the change, and in what way does this paint bottle up light ? To enable me to answer this question satisfactorily, I will carry out another experiment. Here is a pendulum timed to make a complete forward and backward oscillation in exactly three seconds of time. The weight is a very heavy one, of lead, weighing many pounds. What I am going to do is to send little puffs of air from my mouth against the heavy weight, at definite intervals of time, as measured by the small chronometer which I hold in my hand. I shall, to begin with, deliver puffs against the weight, for a minute, at intervals of one second. I shall then change the interval to puffs one in three seconds. [The lecturer then began the experiment. After the lapse of one minute of puffs at intervals of one second, the pendulum was quite stationary and unaffected. After the lapse of the second one minute (puffs at one in three seconds) the pendulum was vibrating freely.] What we have learned is that this pendulum will have nothing to do with any points of air coming at other intervals than one in three seconds, an interval of time which corresponds to its own time of vibration. The energy in these puffs it takes up and accumulates, and it is easy to see, were a minute pair of bellows connected with this swinging weight, air puffs might be obtained from the stored energy. Now, for this pendulum, substitute the vibrating atoms of a sensitive body, and for the puffs of air substitute the beats of the light waves, and we have a complete explanation of the phenomenon known as sensitiveness to light. The motion of the atoms of sensitive substances is altered by the beats of the light waves.—Instead of a pendulum beating one and a half seconds, and beats of one every three seconds, we come to deal with beats of light of many millions of millions per second. We have noticed, while looking at the luminous plate, that the light given out is not of the same colour as the light with which the plate was excited. Those light waves in the mixed light whose periodicity corresponds to the times of vibration of the atoms, are absorbed. When the plate is placed in the dark, these atoms give out the particular light which they have absorbed, and that light only. Now the action on a photographic plate is something quite similar to these we have been considering, only our eyes are not suited to observe the effects of light on the silver salts. There are many sounds in nature that we cannot hear, and there are lights we cannot see. After exposure in the camera, the places on the plate that have had most light have the atomic motion most developed. The light puffs on the vibrating atoms have been strongest there, and consequently they have gathered up more energy than the rest of the plate, and are oscillating between greater extremes. This, then, is of what the photographic latent image consists : a greater or less motion of the atoms of the sensitive substance, those portions having had the greatest amount of light having the greatest motion. It must be remembered, however, if light is allowed to act for a long time on a silver salt, an actual decomposition takes place —a “ printing out ” action. This may be seen to perfection when ordinary printing paper is exposed for a long time under a nega tive. Of these actions we are not treating at present, but of the more mysterious action in the camera when exposures, through small openings, are of a few seconds only at longest. It is not a true decomposition which is effected, but the atoms of the sensitive substance are thrown into a state of vibration, which renders them easier of decomposition by outside agency. And now a word about the action of the developer. It matters not whether we are operating with sulphate of iron on a collodion plate, or with oxalate of iron, or alkaline pyrogallic on a bromide plate ; the action is the same. These bodies have an intense affinity for oxygen. The common soda pyrogallic developer of the photographer is used in scientific laboratories for the absorption of oxygen in gas analysis. What happens during development is this. In those parts of the plate where the atoms are oscillating between the greatest extremes—that is, the portion most acted on by light where, therefore, the bromine and silver have the least hold on each other, an action begins which consists in the developer seizing the bromine and leaving the silver in the film to form the image. It is deposited in the film, not in proportion to the actual amount of light which reaches the plate, but exactly in proportion to the amount of light of a given rate of heat. The red light beats are too long; they do not move the atoms. The blue and violet have about the right length, and it is just in proportion to the movement caused by these rays that the image is formed, apparently untrue to natural tints and lights as we have them in nature. CHOOSING A SUBJECT. BY RANDALL SPAULDING. BEGINNEIS in amateur photography are now so numerous in all our communities as to warrant a word of advice even on a very simple matter. The first picture to be made is of course the family group. The tyro, who has been in quite too much haste to read Robinson and other authorities, succeeds in displaying through the fogs and stains of his first negative some original and decidedly unique posing. The members of the family them selves become quite interested in the novel results, and will bear the strain of repeated trials until the first box of plates is exhausted. The second subject to be attacked will be the amateur’s residence, including the front yard with its trees and flowers. In this the amateur finds a much easier task, provided his camera carries a swing-back and a rectilinear lens. Without these ac companiments he will be doomed to disappointment. The writer was invited two or three days ago to criticise a picture in the making of which the amateur had used a very cheap lens and had neglected to use his swing-back. The rather tall chimney was in imminent danger of falling upon the roof, a situation at first startling, then extremely ludicrous ; I scarcely know myself how this remarkable result could have been brought about. But most amateur's are wise in paying early attention to landscaping ; and it is on this point chiefly that I wish to speak. The beginner is, not unfrequently, too ambitious in his first attempts. Her tries to reproduce the most extensive areas i seeks the highest points that afford a view of the whole neigh bourhood and town. I have been often urged to make an ex-
