diminished, and very excellent representations were produced of all natural colours. The main point to aim at in the preparation of the plate seems to be to obtain a very sensitive film without any, or, at all events, with the least possible, "grain" in the sensitive salt. A formula published by Lumière seems to attain this object. tives, but when held at an angle to the light the colours are vivid. Viewed directly, the developed images appear like ordinary negaThey are not pure monochromatic colours, but have very much the quality of colours obtained by polarized light. It appears that they are produced by what may be termed "nodes" of differentcoloured lights acting within the film. Thus in photographing the spectrum, rays penetrate to the reflecting mercury and are reflected back from it, and these, with the incident waves of light, form nodes where no motion exists, in a somewhat similar way to those obtained in a cord stretched between two points when plucked. In the negative these nodal points are found in the thickness of the silver deposit. When white light is sent through the film after the image has been developed, theoretically only rays of the wavelengths which formed these nodes are reflected to the eye, and thus we get an impression of colour. produced the spectrum colours, but it was found better to heat | appropriate dyes (orthochromatic plates); the exposure was much the plate till it assumed a rose tint. At a later date Niepce de St Victor chlorinized by chloride of lime, and made the surface more sensitive by applying a solution of lead chloride in dextrin. G. W. Simpson also obtained coloured images on silver chloride emulsion in collodion, but they were less vivid and satisfactory than those obtained on daguerreotype plates. Poitevin obtained coloured images on ordinary silver chloride paper by preparing it in the usual manner and washing it and exposing it to light. It was afterwards treated with a solution of potassium bichromate and cupric sulphate, and dried in darkness. Sheets so prepared gave coloured images from coloured pictures, which he stated could be fixed by sulphuric acid (Comptes rendus, 1868, 61, p. 11). In the Bulletin de la Société Française (1874) Colonel St Florent described experiments which he made with the same object. He immersed ordinary or albuminized paper in silver nitrate and afterwards plunged it into a solution of uranium nitrate and zinc chloride acidulated with hydrochloric acid; it was then exposed to light till it took a violet, blue or lavender tint. Before exposure the paper was floated on a solution of mercuric nitrate, its surface dried, and exposed to a coloured image. It is supposed-though it is very doubtful if it be so-that the nature of the chloride used to obtain the silver chloride has a great effect on the colours impressed; and Niepce in 1857 made some observations on the relationship which seemed to exist between the coloured flames produced by the metal and the colour impressed on a plate prepared with a chloride of such a metal. In 1880 Abney showed that the production of colour really resulted from the oxidation of the chloride that was coloured by light. Plates immersed in a solution of hydrogen peroxide took the colours of the spectrum much more rapidly than when not immersed, and the size of the molecules seemed to regulate the colour. He further stated that the whole of the spectrum colours might be derived from a mixture of two or at most three sizes of molecules. In 1841, Robert Hunt published some results of colour-photography by means of silver fluoride. A paper was washed with silver nitrate and with sodium fluoride, and afterwards exposed to the spectrum. The action of the spectrum commenced at the centre of the yellow ray and rapidly proceeded upwards, arriving at its maximum in the blue ray. As far as the indigo the action was uniform, whilst in the violet the paper took a brown tint. When it was previously exposed, however, a yellow space was occupied where the yellow rays had acted, a green band where the green had acted, whilst in the blue and indigo it took an intense blue, and over the violet there was a ruddy brown. In reference to these coloured images on paper it must not be forgotten that pure salts of silver are not being dealt with as a rule. An organic salt of silver is usually mixed with silver chloride paper, the organic salt being due to the sizing of the paper, which towards the red end of the spectrum is usually more sensitive than the chloride. If a piece of ordinary silver chloride paper is exposed to the spectrum till an impression is made, it will usually be found that the blue colour of the darkened chloride is mixed with that due to the coloration of the darkened organic compound of silver in the violet region, whereas in the blue and green this organic compound is alone affected, and is of a different colour from that of the darkened mixed chloride and organic compound. This naturally gives an impression that the different rays yield different tints, whereas this result is simply owing to the different range of sensitiveness of the bodies. In the case of the silver chlorinized plate and of true collodio-chloride, in which no organic salt has been dissolved, we have a true coloration by the spectrum. At present there is no means of permanently fixing the coloured images which have been obtained, the effect of light being to destroy them. If protected from oxygen they last longer than if they have free access to it, as is the case when the surface is exposed to the air. A method devised by Gabrielle Lippmann, of Paris, by which the natural colours of objects are reproduced by means of interference, may be briefly described as follows: A sensitive plate is placed in contact with a film of mercury, and the exposure to the spectrum, or to the image of coloured objects to be photographed, is made through the back of the plate. On development, the image appears coloured when viewed at one particular angle, the colours being approximately those of the object. The necessary exposure to produce this result was very prolonged in the first experiments in which the spectrum was photographed, and a longer exposure had to be given to the red than was required for the blue. Lippmann at first employed collodion dry plates, prepared, it is believed, with albumen, and it required considerable manipulation to bring out the colours correctly, A. Lumière used gelatin plates dyed with Action of Light on Chemical Compounds. the chemical action of light. In 1777 Karl Wilhelm Scheele Reference has been made above to early investigations on (Hunt's Researches in Light) made the following experiments on silver salts: "I precipitated a solution of silver by sal-ammoniac; then I' edulcorated it and dried the precipitate and exposed it to the and repeated the same several times. Hereupon I poured some beams of the sun for two weeks; after which I stirred the powder, caustic spirit of sal-ammoniac (strong ammonia) on this, in all appearance, black powder, and set it by for digestion. This menstruum dissolved a quantity of luna cornua (horn silver), though some black powder remained undissolved. The powder having been washed was, for the greater part, dissolved by a pure acid of nitre (nitric acid), which, by the operation, acquired volatility. This solution I precipitated again by means of sal-ammoniac into horn silver. Hence it follows that the blackness which the luna cornua acquires from the sun's light, and likewise the solution of silver poured on chalk, is silver by reduction. . . . I mixed so much of distilled water with well-edulcorated horn silver as would just cover this powder. The half of this mixture I poured into a white crystal phial, exposed it to the beams of the sun, and shook it several times each day; the other half I set in a dark place. After having exposed the one mixture during the space of two weeks, I filtrated the water standing over the horn silver, grown already black; I let some of this water fall by drops in a solution of silver, which was immediately precipitated into horn silver." This, as far as we know, is the first intimation of the reducing action of light. From this it is evident that Scheele had found that the silver chloride was decomposed by the action of light liberating some form of chlorine. Others have repeated these experiments and found that chlorine is really liberated from the chloride; but it is necessary that some body should be present which would absorb the chlorine, or, at all events, that the chlorine should be free to escape. A tube of dried silver chloride, sealed up in vacuɔ, will not discolour in the light, but keeps its ordinary white colour. A pretty experiment is to seal up in vacuo, at one end of a bent tube, perfectly dry chloride, and at the other a drop of mercury. The mercury vapour volatilizes to a certain extent and fills the tube. When exposed to light chlorine is liberated from the chloride, and calomel forms on the sides of the tube. In this case the chloride darkens. Again, dried chloride sealed up in dry hydrogen discolours, owing to the combination of the chlorine with the hydrogen. Poitevin and H. W. Vogel first enunciated the law that for the reduction by light of the haloid salts of silver halogen absorbents were necessary, and it was by following out this law that the present rapidity in obtaining camera images has been rendered possible. To put it briefly, then, the visible action of light is a reducing action, which is aided by or entirely due to the fact that other bodies are present which will absorb the halogens. In the above we have alluded to the visible results on silver salts. It by no means follows that the exposure of a silver salt to light for such a brief period as to leave no visible effect must be due to the same effect, that is, that any of the molecules are absolutely reduced or split up by the light. That this or some other action takes place is shown by the fact that the silver salt is capable of alkaline development, that is, the particles which have suffered change in their molecules can be reduced | have become the foundation of nearly all subsequent researches 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. 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. 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 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 sensitizer. 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 sensitizer" were present. The term optical sensitizer 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 Spectrum Effects on Silver Compounds.-The next inquiry is that, in many cases at all events, the dyes were themselves as to the effect of the spectrum on the different silver compounds. reduced by light, thus acting as nuclei on which the silver could We have already described Seebeck's (1810) experiments on be deposited. He further showed that even when the haloid silver chloride with the spectrum whereby he obtained coloured soluble salt was in excess the same character of spectrum was photographs, but Scheele in 1777 allowed a spectrum to fall on produced as when the silver nitrate was in excess, though the the same material, and found that it blackened much more exposure had to be prolonged. This action he concluded was readily in the violet rays than in any other. Senebier's experi- | due to the dye. ments have been already quoted. We merely mention these Hh G FÉ DCBA 10 GREY = LIMIT OF SPECTRUM AgI+AgNO3 on paper. AgCl+AgNO3 on paper AgI+AgNO3 in albumen Correct Rendering of Colours in Monochrome.-In Plate IV., fig. P. P. P. AgI prepared in bath, treated with KI, D. 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 washed, redipped in silver bath, de- (le.) or in a print by different depths of greys. Abney veloped with pyrogallic acid. Grey AgBr in gelatin, developed alka- D. (1.e.) 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 alkaline ferrous oxalate or acid de- (1.e.) 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 re duced to of its original value, and so with the other glasses The luminosity of the light coming through each small glass squar can be made equal by rotating in front of them a disk in which apertures are cut corresponding to the reduction required. Th 3AgI+ AgBr + AgNO3 collodion, wet plate, acid or alkaline developer FIG. 1.-Spectrum Effects on Salts of Silver. [P.=print; D. developed; l.e. = long exposure]. 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 See Abney, "Destruction of the Photographic Image," Phil. Mag. (1878), vol. v.; also Proc. Roy. Soc. (1878), vol. xxvii. blue glass, for instance, would not be covered by the disk at all, while opposite the white square the disk would have an aperture of an angle of 18°. When a plate is exposed behind the row of glass squares, with the light passing through the rotating disk, having the appropriate apertures for each glass, the negative obtained would under ordinary conditions, show square patches of very different opacity. A light-filter of some transparent colour, if placed in the path of the light, will alter the opacities, and eventually one can be found which will only allow such coloured light to be transmitted as will cause all the opacities in the negative to be the same. As the luminosities of the white light passing through the glasses are made equal, and as the photographic deposits are also rendered equal, this light-filter, if used in front of the camera lens, will render all coloured objects in correct monochrome luminosity. Another plan, based on the same principles, is to place segments of annuluses of vermilion, chrome yellow, emerald green, French blue and white on a disk, and to complete the annuluses with black segments, the amount of black depending on the luminosity of the pigments, which can be readily measured. When the disk is rotated, rings of colour, modified in brightness by black, are seen, and each ring will be of the same luminosity. As before, a screen (light-filter) to be used in front of the lens must be found which will cause the developed images of all the rings to appear of equal opacity. It must be remembered that the light in which the object is to be photographed must be the same as that in which the luminosity of the glasses or pigments is measured. Action of the Spectrum on Chromic Salts.-The salts most usually employed in photography are the bichromates of the alkalis. The result of spectrum action is confined to its own most refrangible end, commencing in the ultra-violet and reaching as far as in the solar spectrum. Fig. 2 shows the relative action of | potassium and ferric chloride. If these two be brushed over paper, and the paper be then exposed to a bright solar spectrum, action is exhibited into the infra-red region. This is one of the few instances in which these light-waves of low refrangibility are capable of producing any effect. The colour of this solution is a muddy green, and analysis shows that it cuts off these rays as well as generally absorbs those of higher refrangibility. Action of Light on Uranium.-The salts of uranium are affected by light in the presence of organic matter, and they too are only acted upon by those rays which they absorb. Thus nitrate of | uranium, which shows, too, absorption-bands in the green blue, is affected more where these occur than in any other portion of the spectrum. Some salts of mercury, gold, copper, lead, manganese, molybdenum, platinum, vanadium, are affected by light, but in a less degree than those which we have discussed. In the organic world there are very few substances which do not change by the continuous action of light, and it will be found that as a rule they are affected by the blue end of the spectrum rather than by the red end (see PHOTOCHEMISTRY). The following table gives the names of the observers of the action of light on different substances, with the date of publication of the several observations. It is nearly identical with one given by Dr Eder in his Geschichte der Photo-Chemie. Substance. Silver. Observer. Date. with J. H. Schulze . 1727 Chloride J. B. Beccarius 1757 No.1 Chloride in the spectrum Scheele. 1777 Chloride photographically used Chloride blackened Wedgwood 1802 Lassaigne 1839 FIG. 2.-The top letters have reference to the Fraunhofer lines; the bottom letters are the initials of the colours. The relative sensitiveness is shown by the height of the curve above the base-line. the various parts of the spectrum on potassium bichromate. If other bichromates are employed, the action will be found to be tolerably well represented by the figures. No. 1 is the effect of a long exposure, No. 2 of a shorter one. It should be noticed that the solution of potassium bichromate absorbs those rays alone which are effective in altering the bichromate. This change is only possible in the presence of organic matter of some kind, such as gelatin or albumen. Action of the Spectrum on Asphaltum.-This seems to be continued into and below the red, the blue rays, however, are the most effective. The action of light on this body is to render it less soluble in its usual solvents. Action of the Spectrum on Salts of Iron.-The commonest ferric salt in use is the oxalate, by which the beautiful platinotype prints are produced.. We give this as a representation (fig. 3) of Davy 1814 Daguerre 1839 metallic silver). Iodide photographically used Herschel 1840 Talbot. 1841 Hunt 1844 1840 1826 1840 Grotthus Hess Mitscherlich 1827 Bergmann. 1779 Chromate Carbonate Oxalate. Benzoate Citrate Kinate Pyrophosphate Stromeyer 1830 Pelouze and Gay 1833 |