If the plates have been made in large sizes, it will be necessary to cut them to the various smaller dimensions required. Some workers cut the plate on the film side, while others prefer to cut from the back. The better plan is to first cut the gelatine film with a sharp knife, and after wards turn the plate over and divide the glass with a good glazier's diamond. Every precaution should be taken to avoid detaching the film from the glass, and to prevent the fine glass powder or chips produced in cutting from getting on to the surface of the film and producing pinholes.
Before packing the plates they are carefuly examined and those which have markings, air bubbles, or other defects are put aside.
Air bubbles are usually caused in the pouring out of the emulsion. The vessel containing the emulsion should be held as near to the plate as possible.
Frilling of the film is sometimes due to unequal drying. If plates be dried in a box that is imperfectly ventilated, the outside parts become dry, leaving a patch of undried emulsion in the center. At the junction of the two parts frilling may be looked for. In warm weather plates are more likely to frill than in cold, as the emulsion remains upon the plate in a liquid state for a longer time, the silver salts held in suspension in the emulsion to fall down and settle on the glass, thus diminishing the adhesive surface. Plates that frill easily may be used with safety if coated with a 'plain collodion made up with Pyroxyline (tough) 6 grains Alcohol (.820) yi ounce Ether (.725) 5 ounce immediately preceding development, the solvents being washed away in a dish of clean water, and the developer applied; or the plates may be coated with the collodion and allowed to dry if they be immersed in one part of ether and three parts of alcohol previous to development.
Opaque spots found on the film are generally due to dust settling on the film in drying.
Comet-like and other peculiar markings are due to irregular drying. Precautions should be taken that a perfectly even current of air passes through the drying box.
There are several methods of packing gelatine dry plates. The most simple way is to place them face to face with little frames, made the same size as the plate, of strips of cardboard about in. wide. All the different methods have really the same end, i. e., to keep the sensitive films from touching each other. For those who do not manufacture large quantities light-tight boxes with grooves to hold the plates will serve every purpose.
DUST.—Present in the atmosphere from various causes. Its existence is most easily seen when a beam of light traverses the air, lighting up the suspended molecules. Aiken has devised
an apparatus for counting the number of dust particles in sample of air or gas.
Dust is a great nuisance to the photographer, producing black or white specks in the posi tive or negative. Before a dry plate is laid in the dark slide it should be dusted with a camel hair brush, or if any dust be adhering to it during exposure it will show in the negatives as tiny white spots and in the positive as black ones. The negative should also be dusted before placing it in the printing-frame, or the same result will occur, except that this time we obtain little white spots in the positive. In the manufacture of photographic plates and paper the utmost precau tions must be taken to prevent dust settling on and adhering to the wet films.
chromatic or isochromatic process, as sensitizers to the yellow and orange rays. The principal of these are coralline, aldehyde green, chinolin, eosin, erythrosin, and cyanin. (See Orthochro matic Photography.) The use of the dye as a sensitizer is more clearly explained under Orthochromatic process. It might be as well here to mention, however, that errors have often been made by writers in stating that red dyes render the plate sensitive to red rays, green dyes to green rays, and so on. Many dyes act quite to the contrary. Fuchsine or magdala red cause bromide of silver to be sensitive to yellowish green and not to red, while aldehyde green sensitizes for red rays. The general rule is that dyes make bromide of silver sensitive to those rays which they absorb, although all dyes do not act in this manner. Those dyes which bleach rapidly in the light are usually the best, although there are many exceptions to this rule. Triphenylic red, fuchsine, eosine, cyanine, picrate of methyl. magdala green, carmine, annatto, turmeric, chloro phyl, methyl violet, alkaline blue, rosaniline and aldehyde green have very peculiar effects. If these dyes be mixed with bromide or chloride of silver, and exposed to light, the bromide or chloride of silver shows an increased sensitiveness to those parts where the absorption spectra of the respective dyes lie; the action being only visible if certain substances which serve as sensi tizers are present, such as silver nitrate, tannin, pyrogallic acid, etc.* DYE SENSITIZERS.—Dyes used as sensitizers in the orthochromatic process, q. v.
DYNACTINOMETER.—An instrument for measuring the intensity of actinic power, or for comparing the rapidity of lenses.