256. If entrances and exits cannot be made without admission of extraneous light to the dark-room, it is convenient to be able to pass slides containing exposed plates from the studio to the dark-room, or re-loaded slides from the dark-room to the studio, without having to may be wedged in the window frame or secured by screws and bushes.
255. Passages. Where there is ample room, it is a great convenience to provide means for entering and leaving the dark-room without admitting light. One method is the " drum," as shown in Fig. 151, the vanes closing on one another for ventilation when the dark-room is not in use. Another plan is a series of partitions represented in Fig. 152, the centre partition being preferably movable to admit large objects. If neither of these means are possible, one must be content with an ordinary door (Fig. 153) opening into an enclosure, covered with a large opaque black cloth, hanging in folds and arranged if possible against a frame which forms a species of rebate. Whatever the means adopted, the walls of the " passage " must be painted matt black to absorb as completely as possible any light that may reach them.
admit white light to the dark-room. An arrange ment as shown in Fig. 154 can be built in the partition for this purpose. It consists of a wooden box with a false bottom and two openings opposite each other ; a flexible curtain of strips of wood mounted on cloth is arranged so that one opening cannot be un covered until the other has been completely closed.
Another device for the same purpose is shown in Fig. 155. Within the thickness of a partition wall a kind of cupboard is arranged, closed on each side by a guillotine or sliding door. A cylindrical iron rod, sliding in grooves, forms a common bolt between the two doors, one of the latter being able to slide only after a peg in the bolt has been fitted into a hollow in the other door, which is thus locked. When one door is open the bolt cannot be disengaged from the other, since it is flush with the inside wall of the open door.
Use has been made for the same purpose of a drawer opening from either side of the partition, and sliding in a sheath long enough to cover it entirely.
257. Ventilation. Except in the case of dark
rooms, having permanent entrances by " pas sages," the exclusion of all light obviously necessitates the closing of all the means of venti lation usually found in a room. It is therefore necessary to arrange a system of ventilation which does not allow the passage of light, by air-ways to another room, or preferably to the open air.
In either case, two systems arc required, one for the entrance of fresh air, and the other for the removal of the contaminated air, the former near the floor of the dark-room and the latter near the ceiling.
When the ventilation is taken from a corridor or from another room, the partition is provided with a wooden frame similar to that shown in section in Fig. 156. The passages are formed by thin sheets of wood or metal mounted in the frame. When the ventilation is taken from the open, the wall is fitted with hollow bricks (Fig. 157), the passage being made on the outside by a zinc baffle plate, and, on the inside, by a wooden frame with a wood or iron panel. In both cases all inside walls of the passages are painted matt black to avoid successive reflections of light.
Ventilation to the open may also be made by shafts or wide tubes of wood or sheet iron with knees, so as to provide exclusion of light.
258. Heating. The temperature of the baths used in photography considerably affects the speed of the reactions and the quality of the images. The use of time methods of working requires the temperature of the developer to remain practically constant from one operation to another, or at least during a single operation. It is useless to try to use a warm developer in a cold dark-room, or vice versa, except by em ploying a water-bath of large capacity.
The best way is to keep the temperature of the dark-room constant within narrow limits (about 65' F.). If central heating is not avail able, the flue of a stove in a neighbouring room may be arranged to pass through the dark-room. A closed combustion stove in the dark-room is a mistake, on account of thc dust produced in charging and discharging. Failing other means, an electric heater may be used ; unfortunately, it is a very expensive mode of heating.