This should be in an elevated position, the south side entirely closed, the north side being glazed with tolerably thick glass, as free from colour as possible, but preferably of a blue tint, to anything at all approaching green or yellow, as these colours, by neutralising the light, tend to prolong the photographic operations. If possible the length of the room should run from east to west, and the ends be protected from the morning and afternoon sun. A room lighted only from the north side has the softest and most uniform light that can be obtained. Part of the roof may be glassed, and curtains of a bluish colour should be fixed, with an arrangement of cords and pulleys, by which they may easily be adjusted to admit light, or cast a shadow in the required direction. The colours of the wall must be carefully chosen, avoiding red, yellow or green; a bluish grey is the safest, and may be used of several tints to give variety. Movable backgrounds painted in different depths of colour are useful to modify the result of any ill-chosen colours worn by the sitter. Oil colour must be avoided for walls or .backgrounds; a mixture of slaked lime, litmus, or lampblack may be employed, varying the quantity of lampblack to give the required shade.
In the choice of dress, the sitter must remember that cold colours, such as blue or violet, come out white in photographs, whilst the warn colours, red, orange, or yellow, give various shades of black. Articles of dress with vertical stripes tend to give an appearance of increased height to the portrait. The sitter should assume an easy natural position, avoiding a direct vertical light, which falling on the top of the head gives to dark glossy hair the appearance of greyness, and throws very heavy shadows under the eyes, nose, and chin. The best position is a little back from under the skylight, with the head slightly retiring from the side light. The whole figure is then well illuminated; the deepest shadow on the face will be on the retiring cheek, in a three-quarter view, which is generally the best to take. The partial profile will be clearly defined on the shadowed cheek. The position of the body in relation to the head is a matter of taste, The distance of the figure from the background, and its height on the plate, are points which must be regulated by the artistic skill of the operator.
If the sitter is placed several feet in front of the screen, the picture will have greater relief, and the apparent height of a person is much affected by the position of the portrait on the plate. Avoid over crowding the background with vases, columns, and curtains, or anything which will divert the attention from the principal object; as a rule a plain background is the best, the introduction of superfluous furniture and ornamentation most frequently gives a photograph an unpleasant tone of vulgarity. If the headrest is used, it must be carefully adapted to the head, which should only lightly press on it. When the position is settled and the focus arranged, the sitter should not alter his attitude, though perfect immobility is unnecessary. When the operator has the plate ready to expose, he should caution the sitter to keep the eyes fixed in one direction, and to remain perfectly steady; he may then uncover the lens. The nearer the camera is brought to the sitter, the longer the exposure; thus the time of exposure may be varied from one second to 300 seconds. As a general rule, for a full-length figure, in summer, the plate should be exposed 20 seconds; a sitting portrait will require 30 seconds.
In winter the exposure must be increased in duration one-half. Dark Boom. - During certain parts of the process it is imperative that the operator should work in a room into which not a ray of direct light is admitted. This is usually effected by closing every window but one, and that is carefully obscured by yellow or orange coloured curtains, or calico cloth, or a second window sash may be glazed with dark yellow glass. Lamps or candles, provided with yellow screens, may also be used. The dark room should not be too small, as in it several important operations have to be performed; it should be fitted up with shelves for chemicals, a sink and tap, with a good supply of water, several pails for refuse slops, jugs, and draining stands for the plates. The room should be well ventilated, the door and window being kept open as much as possible when the room is not in use, provided that the weather is not too cold, as an even and tolerably-warm temperature is necessary for the proper working of the photographic chemicals. In winter the room must be kept warm; gas or charcoal stoves for this purpose 'should, however, be avoided.
Keep the room as clean and free from dust as possible, and place over the bottles of chemicals small covers of paper, twisted round like an extinguisher, to keep the dust from the necks and stoppers. (See also iv. 403.)