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light, plates, yellow, rays, sensitive, dark, dry and air

DAIIMAR (Malay Damar).—A pale yellow or white resin obtained from the Dammara Australis, commonly known as the kawrie or cowdie pine in New Zealand, and the Damnzara alba, Dammar pine, or cat's-eye resin of India. Dammar resin is also obtained from the Amborgna pine growing in the Malay Archipelago. It is largely used in the manufacture of varnishes, and is easily soluble in oil of turpentine benzole, or chloroform. A dilute solution of gum dammar in turpentine makes a good retouching medium. See Varnish.

DAMP.—Moisture or dampness is a silent and secret to the photographer. In a damp atmosphere nearly all the photographic preparations will suffer. It is, therefore, necessary to keep the chamber containing them in a dry state. This may be done by the application of heat in the damp weather, or by using calcium chloride, which effectually absorbs moisture from the atmosphere.

Damp walls are not conducive to longevity of silver prints hung against them. In such cases precaution should be taken to back the frames with good stout waterproof paper.

DARK R0011.—The name given to the room in which all sensitive materials are handled. The term is somewhat erroneous, as it is not necessary that the room should be dark in a usual, but in a chemical, sense only. To explain this—We know that dry plates are very sensitive to light, but only certain rays of light are capable of making any change. We also know that a ray of white light passed through a prism can be broken up into a band of colors known the spectrum band, and their order is always the same, i.e. violet, indigo, blue, green, yellow, orange and red. The first are termed rays of high refrangibility, and those at the end, rays of low refrangibility. In photography we find that the change that takes place with sensitive plates is produced almost entirely by the rays of high refrangibility, the violet and blue having the most powerful effect, whilst the rays of low refrangibility, as yellow and red, have comparatively no effect whatever. The first are therefore termed actinic, and the last non-actinic.

It is this discovery which enables us to handle the sensitive plates in a room lighted only by non-actinic light without fear of spoiling them.

The question as to which is the best color for the light is a disputed one. It is evident that what we require is a medium which shall give the brightest possible light to work with, but will at the same time be perfectly non-actinic, and have no effect upon sensitive dry plates exposed to it for some time.

A little reflection will show that this depends to a great extent upon the sensitiveness of the material with which one operates. For instance, for the wet or dry collodion processes, gelatino-chloride plates, or paper, a canary-yellow glass will be sufficient protection from the actinic rays of daylight. For slow plates and for bromide enlarging two thicknesses of canary yellow medium will suffice; while for rapid gelatino-bromide plates a ruby-red glass will perhaps be necessary; although two thicknesses of orange paper should be sufficiently non-actinic for the most rapid plates, if they are not exposed to it for too long a period, as no light is absolbtely safe, bat will affect a gelatine dry plate if sufficient time be allowed it to act. The yellow glass is considered superior and safer than red, but it is necessary that precautions be taken to avoid using a yellow medium which will allow too much green light to pass through it.

Besides the quality of the light the quantity must also be considered. Daylight should never be recommended, as it is too variable. The window in the room should be blocked up with two or three thicknesses of brown paper, and artificial light employed. A large number of different dark-room lamps have been devised for this purpose.

The dark room should be as large as conveniently possible. It is a great mistake to suppose that any little closet or cupboard will suffice, and for those who require to be in it for prolonged periods it is extremely dangerous th their health. The room should be fitted with a sink having a tap above it, and several tables and shelves. Fig. 149 is a plan of a dark room with convenient fittings.* Another important consideration is the ventilation of the room. It should be as lofty as possible, and an outlet for the vitiated air should be made in the ceiling or high up in one of the walls. It must be so arranged that it will let out the air without letting in light. Fig. 15o explains how this is done.

Besides this there should also be an inlet for the fresh air. This may be provided by piercing some holes at the bottom of the door and attaching a' piece of wood outside in such .a position, as shown in Fig. z5 z, as to prevent light from coming in, but without pre venting the entrance of pure air.

The dark room should be kept perfectly clean, and everything cleared up and put into its proper place after every operation. Have a place for everything, and keep them all in their places.