The correct rendering of colour by tones should have careful consideration. The term tone here must not be confounded with the same term used in P.O.P. toning. It really means the suggestion of colour by means of varying depths of deposits in the picture. Blue photographs white; if the subject of the picture be something white and has for its background something .blue, the difference between the two will be so slight as to afford no contrast. Again, in the case of a yellow flower if photographed in the ordinary way, it will come very much darker than it appears naturally. Flesh tints suffer also from the same cause. These difficulties are overcome to a certain extent by means of what are called "Orthochromatic" plates. These are plates that have been chemically treated to render them more sensitive to the less active rays of light as yellow, green and red. The orthochromatic plate is used in conjunction with a yellow screen or filter, which materially lengthens the necessary exposure according to the depth of the colour of the screen; from three to six times more exposure will be required with it than without it. The screen is fitted up either inside the camera, at the back of the lens, or into the hood of the lens-mount. Working under these orthochromatic conditions with the white object against the blue background, the blue rays of the background will combine with the yellow of the screen to form a green tint. Green, being less active than blue, would photograph darker and give greater relief to the white. The yellow of the flower would combine with that of the screen and it would come lighter. Any yellowness, as freckles, etc., in a face would be less prominently marked from the same reason. The time of exposure on an orthochromatic plate under ordinary circumstances and without a screen is about the same as non-orthochromatic plates, except at evening time, about sunset, when there is a considerable amount of yellow light about; the ortho-plate is then more rapid than a non-orthochromatic. Orthochromatic plates must be handled with great care in the dark-room, and nothing but ruby light must be used, and the plate should be covered during development by placing a piece of cardboard over the dish.
The colour suggestions by tones or different intensities of deposits in prints from negatives of orthochromatic plates must not be confused by the beginner with photography in colours; in this latter there is yet a wide field for research. The farthest advance to the present has been to get colour renderings on the transparency, that is, a positive on glass instead of on paper. These are produced by superimposing films of yellow, blue and red, one upon the other in exact register. The negatives - from which the positives are made - are obtained on three different plates, rendered specially sensitive to the colours mentioned. These plates are exposed separately in a special camera behind suitably coloured screens, and afterwards developed, fixed, washed and dried. Exposures are then made on specially treated lantern or other plates or films to make the positives, and when finished these are bound up in contact. The resulting blending of the colours produces a picture giving the colours of the subject. These are best seen in a stereoscope, or when thrown on a screen from a lantern.
The recently introduced Autochrome plate has materially simplified the operation of colour photography, as it is only necessary to use one plate instead of three. This plate is specially prepared by first coating the glass with a layer of specially coloured starch-granules; the interstices between the granules are filled with an opaque material. This compels the rays from the object being photographed to pass through the coloured starch granules, which act as a special light filter. This filter is further assisted at the time of exposure by another special screen placed near the lens.
The sensitive emulsion is spread upon the sub-stratum of starch; during the exposure the light has to pass through the glass and starch screen to reach the sensitive material, therefore the plate must be put into the dark-slide sensitive surface inwards, and great care must be taken not to injure this surface. On account of the plate being put reversed ways into the holder, it becomes necessary at the time of focussing to have the focussing screen reversed, that is, the ground surface outwards, to ensure a sharp image. The length of the exposure will be according to the class of subject in hand. Experience in the use of the ordinary plate will be helpful. The necessary exposure for an Autochrome plate may be taken as twice that necessary for an ordinary slow plate.
The development is more complicated than ordinary. It consists of seven distinct stages, and each stage is for a stated time; the plate is washed between each.
The plate is partially developed, then placed in a second bath in which the image is reversed from the negative to the positive state. Further developing is now carried on, and from this bath it is passed to an oxidizing bath and then intensified; after passing through a clearing bath, it is fixed, washed and dried. Finally, it is bound up in contact with a cover glass, after the manner of a lantern-slide.
On account of the sensitiveness of the Autochrome plate to the rays of coloured light, the filling of the dark-slide and the first stage of development should be carried out with as little photographically inactive light as possible, if not in total darkness; after this the further stages of the operation maybe performed in weak daylight.
The starch substratum causes the attachment between the glass and the sensitive film to be only of a very slight character, and unless some precaution is taken during the process of development, the two will part. This may be avoided by edging the plate with some impervious material to resist the moisture, or by the use of a specially constructed dish for the purpose. Still-life subjects, as flowers, fruit, etc., are very suitable for colour photography.