Ill-defined patches of greater density than that of the picture as a whole point to dirt on the glass, which, by preventing access of light to the glass, causes less density in the primary image and consequently greater density in the second development. Local reduction may be resorted to, but it is a risky matter.

A brilliant picture, wanting in details in the highlights, is generally due to too long primary development, or the reversing bath has been used too long. There is no remedy for this. If the whole picture is weak and almost colorless in the shadows as well as the high-lights, then overexposure and too long primary development are the cause. The only thing to do is to intensify repeatedly with the silver intensifier. If the picture looks correct after intensification, but almost disappears in the fixing, either traces of permanganate were left in the film, which reacts with the acid of the bath and acts as a reducer, or the second development was not carried far enough. The best remedy would seem to be physical development with the following:

Ammonium sulphocyanide 24 g.

Silver nitrate 4 g.

Sodium sulphite, dry 12 g.

Hypo 5g.

Potassium bromide 0.5 g.

Distilled water 100 ccm.

For use, add 10 ccm to 80 ccm water and 10 ccm metol-hydrochinon, rodinal or other developer. This solution should be renewed as soon as it gets muddy. The action is rather slow, and if the image appears whitish, when finished, it should be bleached in a one per cent solution of mercuric chloride, washed, and developed with an ordinary developer, diluted.

Failures due to the filter show a more or less blue tinge. If the whole picture is blue or bluish-green, then the filter has been omitted altogether; stray white light in the camera also tends to give this effect. There is no remedy for this. If the filter was used, then one must assume that it has faded somewhat, due to careless undue exposure to strong white light, or if the filter is home-made, then incorrect quantities of the yellow dye have been used.

When using artificial lights, too reddish pictures are due either to the use of an improper filter or to underexposure. Because nearly all artificial lights have an excess of orange rays and are deficient in the blue, the tinge of filters for this work should always be more or less greenish to cut down the excess of orange and red.

Naturally, undue exposure of the plate to an unsafe darkroom light may cause fog. If this is general, it is caused by exposure of the film, whereas if the glass has been unduly presented to the light, the latter may penetrate the screen elements, and then the general fog is either greenish or bluish, according to the color of the safe-light.

One of the most annoying and frequent failures, especially with beginners, is the appearance during the second development of negative patches, when the plate is looked at. This is particularly noticeable in portraits, when patches of black silver make their appearance on the cheeks and hands of the sitter, or sometimes on white objects, such as a man's collar. This is always due to insufficient action of the reversing solution. Local reduction is the only remedy for this.

Small black specks generally arise in the manufacture of the plates, and are practically unavoidable; on the other hand, they do not often occur and are as a rule very small. Should they happen in the shadows they may usually be ignored, but in the highlights they should be touched with a match, sharpened to a point, or with a very fine camel-hair brush, moistened with the reversing solution or the following:

Potassium iodide 3 g.

Iodine 1 g.

Water 50 ccm.

As soon as the spot disappears, immerse the plate in water, then in the fixing bath. These spots are actually metallic silver and the above solution converts them into silver iodide, which dissolves in the hypo.

Green spots are due to injury to the insulating varnish between the screen-element layer and the emulsion, which allows access of water to the former, so that the green dye dissolves. There is practically no remedy except local retouching with water-colors, so as to break up the area, but this is unsatisfactory at its best.

Exhibiting Screen-Plates

Naturally, the use of the screen-plate for lantern slide work presents great attractions and they sometimes form a welcome relief to the black and white variety. On the other hand, they cannot be shown at their best along with ordinary slides, for as a rule the latter are shown on far too large a scale for all screen-plates. The color elements absorb a great deal of light, with the result that, when projected on too large a scale with an ordinary arc or lime light, they look too dark and a great deal of their beauty is lost. It is, of course, a matter of personal opinion, but the author considers that a four-foot picture is the largest that should be shown with screen-plates. With regard to the absorption of the light, this is over ninety per cent with the autochrome and about seventy-five per cent with the Paget plate. There is another factor in the projection of screen-plates and that is the size of the color elements. If we take the diameter of a lantern plate as four inches and project this on a four-foot disc, we have a magnification of twelve diameters. Each color unit is magnified in like degree, so that, if they are too large, they will be distinctly visible. In the case of the autochrome the mean diameter of the elements is 0.015 mm; in the case of the Paget they are squares of about 0.63 mm for the blue, and 0.84 mm for the red and green. If we multiply these by twelve, we obtain 0.18 mm for the autochrome and 1.008 cm for the Paget, and it is obvious that at the normal distance of the first row of the audience, which we may put at ten feet, they will be quite invisible. Practically the nearest distance at which the color element becomes visible at is 1000 times its diameter, therefore one can easily calculate the correct distance for the nearest observers for any degree of enlargement.