DARK-ROOM ILLUMINATION The colour of the light used for dark-room illu mination is important, both as regards its purity and its general character. For ordinary negative work a pure red, that does not allow any blue rays to pass, is best, and for bromide printing a golden yellow or orange will give sufficient safety with comfort in working. For most colour-sensitive plates a pure deep red is best ; but for some that are very sensitive to red a special screen for the dark-room lamp is sold by the makers of the plates. For all ordinary plates and papers the best method for working satis factorily is to light the dark-room brilliantly, sufficiently well for seeing all bottles, measures, plate boxes, etc., clearly and distinctly. Unless there is sufficient light for working in comfort, inferior results are inevitable. The room should be lighted well, but the plate should be shielded or protected from the direct rays of the lamp, both in filling the slides and during develop ment. The plate may be brought near the lamp the insertion of glasses or fabrics of different colours according to the work in hand. With transparent glass a good deal of light is often wasted ; lamps with fabrics capable of diffusing the light, or fitted with sheets of ground glass or opal in addition, for the same purpose are to be preferred, as the room is then more com fortably and uniformly lighted, and the bench, shelves, etc., are more readily perceived. For temporary use, as when travelling, a folding lamp A burning a candle or night-light is con venient ; but for constant work candles are troublesome and uncertain. Oil lamps are fairly satisfactory, if fitted with the wick adjust ment outside, as in B, and kept scrupulously clean ; but gas and electricity are the dilly forms of illumination that can really be recommended. From the facility with which it may be raised or lowered, gas is perhaps the best of all, if perfect ventilation is secured ; and a good pattern of gas lamp is shown at C. A lamp for use with electricity is illustrated at D ; hanging electric lamps are also obtainable. The worker should be warned that many of the red incandescent electric bulbs in the market are unsuitable for for a few seconds, when necessary, for examin ation, but otherwise should be shielded as much as possible. It is not necessary to cover the dish, as the light reflected from the walls has very little power ; but the dish should be z ft.
or 3 ft. from the lamp and shielded from the direct rays. Artificial light is very much, safer for dark-room illumination than daylight, and should always be used when convenient.
LAMP (Fr., Lampe de atoire ; Ger., Dunkelzimmerlampe) On the satisfactory working of the dark-room lamp depends much of the operator's comfort when developing, while the " safety" of the light emitted is an important consideration. This is qualified to a great extent by the kind of plate or paper that is being used, and its particular region of colour sensitiveness. Thus, for wet collodionplates an amber-coloured glass may be used ; for bromide paper, one or two thicknesses of yellow fabric ; for ordinary plates, one thickness of orange fabric and two or three thicknesses of yellow, or an amber and a ruby glass together ; while for orthochromatic plates a very deep ruby or a special shade of green may be employed. Obviously, therefore, the ideal dark-room lamp is one that will permit photographic employment, unless masked with non-actinic fabric.
With regard to the " safe light " used in the lamp, care must be taken that it is suited to the plate. A good way of ascertaining this is to place an unexposed plate in the dark-slide, with the shutter half-drawn, and to expose it close to the lamp for about four minutes. It should then be developed, when, if the light is unsafe, the exposed half will be fogged, and a dear line of demarcation will be evident between it and the unexposed portion. The exact matching of the light to the plate can only be done by photographing the solar spectrum on the latter, and noticing the region of the spec trum which has no photographic effect. It is then possible to obtain a dark-room light of the particular colour, or mixture of colours, to which the plate is insensitive, by bringing the spectroscope into service. Gelatine films, stained with aniline dyes, are much used as safe lights, as their colour may be nicely adjusted when staining. These require to be kept between two plain glass plates in the lamp. Since heat is detrimental to the stained gelatine films, lamps of special construction are made for use with them. Other lamps have glass cells in front, which may be filled with potassium bichromate solution, or a solution of any selected aniline dye. (See also " Bichromate Lamp.")