Page:EB1911 - Volume 21.djvu/520

Rh which have suffered a change in their molecules can be reduced to metallic silver, whilst those which have not been acted upon remain unaltered by the same chemical agency. Two theories have been offered to explain the invisible change which takes place in the salts of silver. One is based on the supposition that the molecules of the salt can rearrange their atoms under the vibrations caused by the ether waves placing them in more unstable positions than they were in before the impact of light took place. This, it is presumed, would allow the developer to separate the atoms of such shaken molecules when it came in contact with them. The other theory is that, as in the case of the visible effects of light, some of the molecules are at once reduced and that the developer finishes the disintegration which the light has begun. In the case of the alkaline development the unaltered molecules next those primarily reduced combine with the reduced silver atom and again form an unstable compound and are in their turn reduced.

The first theory would require some such action as that just mentioned to take place and cause the invisible image formed by the shaking apart of the light-stricken molecules to become visible. It is hard to see why other unacted upon molecules close to those which were made unstable and which have been shaken apart by the developer should themselves be placed in unstable equilibrium and amenable to reduction. In the second theory, called the “chemical theory,” the reduction is perfectly easy to understand. Abney adopts the chemical theory as the balance of unsubstantiated evidence is in its favour. There is another action which seems to occur almost simultaneously when exposure takes place in the absence of an active halogen absorbent, as is the case when the exposure is given in the air, that is, an oxidizing action occurs. The molecules of the altered haloid salts take up oxygen and form oxides. If a sensitive salt be briefly exposed to light and then treated with an oxidizing substance, such as potassium bichromate, potassium permanganate, hydrogen peroxide, ozone, an image is not developed, but remains unaltered, showing that a change has been effected in the compound which under ordinary circumstances is developable. If such an oxidized salt be treated very cautiously with nascent hydrogen, the oxygen is withdrawn and the image is again capable of development. . 1.—Spectrum Effects on Salts of Silver. [P.＝print; D.＝developed, l.e.＝long exposure].

Spectrum Effects on Silver Compounds.—The next inquiry is as to the effect of the spectrum on the different silver compounds. We have already described Seebeck’s (1810) experiments on silver chloride with the spectrum whereby he obtained coloured photographs, but Scheele in 1777 allowed a spectrum to fall on the same material, and found that it blackened much more readily in the violet rays than in any other. Senebier’s experiments have been already quoted. We merely mention these two for their historical interest, and pass on to the study of the action of the spectrum on different compounds by Sir J. Herschel (Phil. Trans., 1840). He describes many experiments, which

have become the foundation of nearly all subsequent researches of the same kind. The effects of the spectrum have been studied by various experimenters since that time, amongst whom we may mention Edmond Becquerel, John William Draper, Alphonse Louis Poitevin, H. W. Vogel, Victor Schumann and W. de W. Abney. Fig. 1 is compiled from a cut which appeared in the ''Proc. Roy. Soc.'' for 1882, and shows the researches made by Abney as regards the action of the spectrum on the three principal haloid salts of silver. No. 7 shows the effect of the spectrum on a peculiar modification of silver bromide made by Abney, which is seen to be sensitive to the infra-red rays.

Effect of Dyes on Sensitive Films.—In 1874 Dr H. W. Vogel of Berlin found that when films were stained with certain dyes and exposed to the spectrum an increased action on development was shown in those parts of the spectrum which the dye absorbed. The dyes which produced this action he called “optical sensitizers,” whilst preservatives which absorbed the halogen liberated by light he called “chemical sensitizers.” A dye might, according to him, be an optical and a chemical sensitize. He further claimed that, if a film were prepared in which the haloid soluble salt was in excess and then dyed, no action took place unless some “chemical sensitize” were present. The term “optical sensitize” seems a misnomer, since it is meant to imply that it renders the salts of silver sensitive to those regions of the spectrum to which they were previously insensitive, merely by the addition of the dye. The idea of the action of dyes was at first combated, but it was soon recognized that such an action did really exist. Abney showed in 1875 that certain dyes combined with silver and formed true coloured organic salts of silver which were sensitive to light; and Dr Robert Amory went so far as to take a spectrum on a combination of silver with eosin, which was one of the dyes experimented upon by J. Waterhouse, who had closely followed Dr Vogel, and proved that the spectrum acted simply on those parts which were absorbed by the compound. Abney further demonstrated that, in many cases at all events, the dyes were themselves reduced by light, thus acting as nuclei on which the silver could be deposited. He further showed that even when the haloid soluble salt was in excess the same character of spectrum was produced as when the silver nitrate was in excess, though the exposure had to be prolonged. This action he concluded was due to the dye.

Correct Rendering of Colours in Monochrome.—In Plate IV., fig. 14 the sensitiveness of a plate stained with homocol is shown, and it is evident that as it is sensitive throughout the visible spectrum there must be some means of cutting off by a transparent screen so much of the spectrum luminosity at different parts that every colour having the same luminosity to the eye shall be shown on a negative of equal density. When this is done the relative luminosities of all colours will be shown by the same relative densities or in a print by different depths of greys. Abney devised a sensitometer which should be used to ascertain the colour of the screen that should be employed. By proper means the luminosity of the light of day coming through a red, a green, a blue and an orange glass can be very accurately measured; if -in. squares of these coloured glasses, together with a white glass of the same area, be placed in a row and cemented on white glass, we have a colour screen which we can make available for finding the kind of light-filter to be employed. This is readily done by reducing the luminosity of the light coming through all the glasses to that of the luminosity of the light coming through the blue glass. If the luminosity of the blue be 5 and that of the white light 100, then the luminosity of the former must be reduced to of its original value, and so with the other glasses. The luminosity of the light coming through each small glass square can be made equal by rotating in front of them a disk in which apertures are cut corresponding to the reduction required. The 