varying circumstances of light--the fickle clouds and the uncertain haze, the meridian glare and the slanting ray of the morning and evening, as well as the summer's glow and the winter's mist—render the time of exposure a matter of exceeding difficulty, which experience only can overcome, and enable one to attain to the exercise of a just judgment. It may be mentioned, however, that an exposure through a screen for a half-tone negative will take about four times that of a negative intended for line work. Before uncapping the lens examine the conditions of the light, and determine what stops you will use, and mark down on a slip of paper the time you intend to give to each, then turn the hands of a striking clock to the time allotted to the first stop, and the instant of its striking will give a perfect warning that time is exactly up, when the lens may be capped and the stop changed, and the clock again set to keep its vigils for the fullness of time.
Exposure an example of approximate timing, we will suppose a picture to be copied, and to be reduced to about one-half. Referring to what has been said regarding stop apertures on p. 408, we would use that marked No. 1, or a round aperture of same size, say f/75, and give seven minutes ; then change to No. 2,f/5o, and give two minutes ; change again to Nos. 3 and 4, same size, and give one minute each ; being eleven minutes in all, with a fairly clear light, through a screen of 12o lines to the inch, and with a rapid rectilinear lens of a reputa ble make. The light, or other circumstances, may call for a different exposure, but this is given as an example that will be found nearly correct.
The a saturated solution of the proto-sulphate of iron, of which take twelve ounces, and add to it two ounces of acetic acid and twenty-four ounces of water. Some add a little alcohol, but this is not necessary unless the silver bath is getting out of order from use, or the accumulation of iodide, alcohol, or organic matter in it, which may be known by a dif- • ficulty in flowing the developer over the plate, and may be accepted as a warning to see after a new bath, and an early rectification of the old.
The Fixing agent used to fix the picture, or destroy the sensitive character of the plate, is the cyanide of potassium, made up in solution in the proportion of one ounce of cyanide to twelve ounces of water. It is highly poisonous and should be carefully kept from any scratches or abrasions on the hands, and so save any risk of blood poisoning.
exposure made, retire with holder to the dark-room, which we have supposed to have been fitted up with sink, water, and the usual necessary equipment of such "dens." The developer and the fixing chemical are supposed to be lying convenient to hand.
Take the plate from the holder, and, holding it by the corner in the left hand, flow the developer with a steady and rather copious sweep over it, yet permitting as little as possible, or none, to overrun. Flow back and forth until the appearance of the image, which, if correctly timed, should be in a few seconds, and watch for the coming of the details of the picture, and on the instant that they are seen to be sufficiently full, stop further development by putting the plate under the tap, and letting the water flow freely over it. Guard against over-development. If allowed to go too far there will be a veil or fog over the plate, which it is difficult to get rid of, with great risk of spoiling the negative in the attempt.
Trimming the development having been carried as far as desired—i.e., with the detail fully out, but no more—and washed with a generous flow of water, it is now ready for being " fixed," that is, the image rendered non-sensitive to the light, which is done by flowing over the plate the before-mentioned solution of cyanide of potassium, when the whole picture will be seen immediately to clear up. Before doing so, however, it is well to trim up the negative by scraping away with the thumb the film from the outer margin of the picture, allowing about an inch or so to remain. As the film is very rich in silver, a small receptacle should be provided, easily found in the dim light of the dark-room, where that portion scraped off may be put and preserved.
Examination of the plate may now be taken into the light and examined through a microscopic focusing glass, and the attention first directed to the condition of the dots seen in the high lights, i.e., the dark portions in the negative. These will have come right, if they almost, but not quite, touch each other, while the dots in the half-tones will be of size vary ing as they tend to light or shade, and the deeper shadows will have smaller dots, but sufficiently pronounced to permit a little diminution in the after process of clearing. Should the high lights not be closed enough, and instead show considerably wide cross lines, then a longer exposure with the larger stop, or possibly increasing the distance a trifle between the screen and plate is indicated. If the dots in the shadows are not sufficiently strong, then a smaller stop, or a longer exposure with the smallest stop used, is the remedy. Keeping these rules in mind, with a little practice, the operator will soon master the principles and attain familiarity in working.