It was mentioned in the opening chapter that the whole scheme of photography depends upon the effect of light on chemicals sensitive to its action.

This action varies according to the colour of the light. White light is composed of several colours blended together, violet, indigo, blue, green, yellow, orange and red, as shown by the rainbow, where the white light of the sun is broken up by the rain-drops into its component parts in precisely the same way as white light is decomposed by a prism. A prism is a piece of glass several inches long, having three equal sides, arranged triangularly. The strongest action on sensitive materials is shown by white and blue light, and gradually falls away through the others until red is reached, which has very little action and is therefore the most suitable for use in the room in which the photographic work is done, such as the handling of plates, films, etc. This room is commonly known as the "Dark-room," and will be fully described shortly.

Prominent among the substances sensitive to light are the Haloid Salts of Silver, as Silver Bromide and Chloride. These two salts give rise to two groups of varying sensitiveness, which might be called, first, the "Dark-room Group," embracing plates, films and bromide papers, as they can be opened and handled with safety in a specially lighted room only; and second, the "Daylight Group," which may be opened in weak daylight, and represented by the printing-out papers as Gelatino-Chloride, Collodio-Chloride, Albumen papers and the Self-toning papers. Platinum paper, in which the sensitiveness depends upon the light's action on salts of Iron and Platinum. Carbon paper, in which the effect is upon Bichromated Gelatine.

The Dark-Room Group

The bromide of silver is thoroughly mixed with a thick solution of gelatine to form what is technically known as the "Emulsion"; this is evenly spread upon glass to make plates and upon celluloid for films.

Plates

Plates are made in several degrees of sensitiveness, the most common being the "Slow, or Ordinary," "Medium," and "Special Rapid."

Relatively speaking, the medium speed plate is twice as fast as the ordinary, and the special rapid twice as fast as the medium, or four times quicker than the ordinary; to put it figuratively, if the special rapid plate required one second exposure on a given subject, the medium speed would require two seconds and the ordinary four seconds. The exposure required by a special rapid is rarely more than the fraction of a second.

The leading sizes of British plates are: 3 1/2" x 2 1/2" 4 1/4"x3 1/4" (1/4-plate), 5 1/2"x3 1/2" (post card), 6 1/2"x4 3/4" (1/2-plate), 8 1/2"x6 1/2" (whole-plate), 10",x8", 12"x10", 15" x12".

Films

Films are generally of one speed only, and that corresponds with that of the special rapid plates. They are usually sold in lengths sufficient for six or twelve exposures, rolled upon spools. A strip of black light-tight paper - just a little wider and a good piece longer than the film - is first fixed to the spool, a portion of this paper is first wound on, the celluloid film is then attached to it and both wound on together. The paper is sufficiently long to wind several times round after the film has ended. The whole is made secure by an adhesive label. This arrangement enables the spool to be placed in position in the camera and set for starting - as will be more fully explained under "Loading the Camera " - in weak daylight.

The exposure sizes of films are 3 1/4" x2 1/4", 3 1/2" x3 1/2", 4 1/4" x3 1/4", 5" x4", 7" x5"- Films are also sold in various cut sizes.

Bromide Papers

The emulsion is spread upon paper. These papers are used for obtaining prints from negatives by exposure to artificial light. The image, as in the case of the exposed plate or film, is latent - that is, it cannot be seen after exposure until after the paper has been passed through a developing solution.

There are two speeds of Bromide paper, "Rapid" and "Slow." The paper commonly known as "Gaslight Paper" is really a slow bromide paper, relying on a long exposure and quick development; hence, it is possible to handle it in very weak artificial light instead of in the dark-room. The surface of the paper is prepared either to give a glossy surface or a dead or matt surface.

Bromide paper is sold in cut sizes similar to those of plates. Post cards are also coated with the emulsion.

The Daylight Group. The Gelatino-Chloride Printing-Out Paper (Shortly Called P.O.P.)

The emulsion in this case is prepared by mixing silver citro-chloride with solution of gelatine. The sensitiveness of P.O.P. is not nearly so great as that of bromide papers, and requires exposing to daylight for some time to properly print out. This variety of P.O.P. is in most frequent use and is sold in various cut sizes. ALBUMENIZED printing-out paper and COLLODIO-CHLORIDE P.O.P. are not in so general use - in the first of these the silver chloride is diffused through albumen (white of egg), and in the second the emulsion is prepared with collodion. CARBON paper is prepared by allowing a gelatine-coated paper to float upon a solution of potassium bichromate. The gelatine absorbs the salt and becomes sensitive to light when dry. PLATINUM paper. The paper is first sized with starch (arrowroot), and is sensitized with solutions of iron and platinum. Further remarks upon sensitive papers will be found under the heading "Printing."