Very little assistance can really be given as to exposure, but with a negative of average density, which will give a good silver print, and using a lens working at F/11 and a Mawson lantern plate at midday in May, ten seconds will give a good black slide.
There is but one little point that has been missed—the diaphragm; always use the largest diaphragm which will give satisfactory definition, this will usually be F/11 or F/16.
Be very careful while exposing not to shake the camera—it is quite sufficient for anyone weighing about eleven or twelve stones to walk across the room to give double outlines.
Daylight is not a constant quantity, and although visually the same on two different days, the actinic power of the light varies enormously; therefore we prefer artificial light.
Precisely the same apparatus can be used for artificial light with one or two additions. In some such arrangement in use the printing frame containing the negative is fastened to the side of a cube sugar box in which a hole is cut.
Opposite to the negative on the other side of the box is placed a sheet of white cardboard bent slightly to the arc of a circle. The lights, etc.—two incandescent gas burners do well with tin reflectors behind them—are placed one on each side of the negative inside the box, so that the light is reflected on to the card and thence on to the negative, and no direct light reaches the negative. Absolutely even illumination, even of a large negative, is thus obtained, and the exposure, using the same conditions as stated for daylight, is only twenty seconds.
Of course, the light may be placed directly behind the negative, but in this case a diffuser, such as a sheet of opal glass, must be placed between light and negative, and even then, unless great care is exercised, uneven illumination of the negative and consequent unequal density of the slide must ensue.
We may use magnesium ribbon, and a diffuser of opal is then necessary, and the ribbon must be kept in motion the whole of the time. Magnesium is objectionable because the particles of magnesia form a voluminous cloud, which tastes and smells unpleasantly and settles down on everything. Still, for those who wish to work with this substance, about 18 inches burnt close to the opal and moved about all over it will be about sufficient to obtain good results under above mentioned conditions. An ordinary oil lamp or gas may also be used, provided the light is diffused.
Only the bromide lantern plates are suitable for reduction, the exposure, especially with the chloride emulsions, being so long as to place them out of court. The chloro-bromide may be used for daylight and magnesium ribbon.
After development and fixing, which may be performed in the developers recommended by the makers of the plates used, the lantern slide must be well washed and cleared in an alum and acid bath, then again well washed and finally given a gentle rub with a piece of cotton wool under the tap, and set up to dry.