The requisites of a good lantern picture are: - (a) Artistic composition, the arranging of the subject in such a manner that as the eye wanders over it its beauties continue to grow, and the imagination receives an unalloyed feast of satisfaction and pleasure. To some extent it is possible to teach the art of composition - at all events, so far that its simplest canons may escape violation - but the capacity of rendering true art is a gift of nature. (b) The technical excellence of the picture, its mechanical production.
The wet process yields the best results in many cases. It is at times somewhat troublesome; still, with care and observation, its difficulties can be overcome. Collodion - that is, pyroxyline dissolved in ether and alcohol - forms the vehicle to receive the sensitive salts, and a collodion that has been iodised some time is necessary, otherwise the high lights of the picture will suffer. Thin gloss cut to the standard size is taken, and after standing some time in sulphuric acid and water, it is carefully dried with a cloth free from soap or other grease. One side of the glass is then coated with the following solution, which must first be filtered through filter paper:
Ammonia liquor 0.880 . 1 oz.
The coating is performed by pouring a small pool in the centre of the plate, then gently inclining it so that it runs to each corner; the excess may be thrown away. So soon as the plates are dry, they are ready for use. With a soft brush carefully dust the prepared surface, flooding it with collodion in a similar manner, but returning the excess to the stock bottle. This is to avoid dust, a serious enemy. During the draining the corner should be kept in contact with the bottle, and the plate gently rocked to avoid a streaki-ness or uneven setting of the collodion. Directly it is sufficiently set - the best test of which is trying the upper corner with the finger - it is steadily and evenly lowered into the sensitising bath by means of a dipper.
A good bath for lantern slides is made as follows:-
Pure nitrate of silver recrystallised . 40 gr.
Distilled water. 1 oz.
rendered slightly acid with C.P. nitric acid, one or two drops of which will be sufficient for 12 oz. solution.
When the bath is mixed, for each 12 oz. add 1/2 oz. of the jodised collodion, and shake very thoroughly; let stand 2 hours, then filter. The bath should now be quite clear and in good working order, but may occasionally be placed in the sun for a few hours, and afterwards filtered, when it will work cleanly until the silver is exhausted. When the plate has been in solution about 2 minutes, it should be slightly moved to help the escape of the solvents, and in about 4 minutes may be examined by yellow light. If the surface is free from greasy lines, it is ready for exposure in the camera.
A good negative is necessary for a successful slide. It should be "plucky," so as to admit of a fair exposure.
A good developer is made as follows :-
Protosulphate iron. 1/2oz.
Acetic acid . 2 „
Honey ... 1 „
Alcohol... 1/4 „
Water... 8 „
Use plenty of developer, and cover the plate in one even wave; never mind spilling a little, though practice will enable you to avoid this. As soon as all detail is well up, thoroughly wash and intensify with-
Pyrogallic acid . 24 gr. Citric acid . 24 „ Acetic acid . 1/4oz.
Water ... 24 „
Enough to cover the plate is taken, to which, immediately before use, a few drops of the silver bath are added.
Do not over-intensify, as the picture does not lose much in fixing, for which operation hyposulphite of soda may be used on the ground of safety, but cyanide of potassium acts more quickly and perhaps more cleanly. Slides by this process are a good color, and do not need toning. A coat of clear varnish improves the transparency of the shadows.
Whether printing in contact or by means of the camera, I strongly recommend a full exposure. Gelatine plates are sure to show fog if forced, and however slight that may be, it should ensure their immediate rejection. Indeed, it is well to select a really good slide as :t standard both as to density, tone, and clearness in the high lights, and those that do not come up to it should not be kept. For dry plates I prefer a soft negative full of detail. If the skies are not sufficiently opaque, they must be stopped out.
The solutions required are 10 per cent, ones of the following: Pyrogallol, bromide of potassium, ammonia, carbonate of ammonia, and carbonate of potash. The pyro is mixed as follows : 4 oz. sulphite of soda are dissolved in boiling water and rendered acid with citric acid. The pyro is then added, and the whole made up to 10 oz. with water. The other chemicals are simply mixed with water, and all will keep well. A developer giving a beautiful purple tone with Mawson's and Thomas' plates is-
Pyro solution . 30 minims. Bromide . 30 „ Ammonia . 30 „ Carbonate ammonia 30 „ Water to make up to 1 oz.
The same colour can be obtained with Fry's plates by slightly increasing the exposure and bromide, while a fine engraving black is got by shortening the exposure, increasing the ammonia, and leaving out the carbonate of ammonia. Sepia is obtained by full exposure and using carbonate of potash or ' soda in place of ammonia; but some makes of plate will not yield the sepia tone.
While the plates are developing, keep them in motion; it adds to their vigour, and avoids Oatness, and prevents deposit settling upon them. After fixing and moderately washing, they may be cleared in-
Alum ... 2 oz.
Citric acid . 1/2 „ Water ... 10 „
The addition of 2 oz. protosulphate of iron and 1/4 oz. more of citric acid will considerably moderate the tone, and by slightly reducing the slide increases the clearness of the high lights. If any deposit appears upon the surface, rub gently with the finger or a tuft of cotton wool. The slide is now well washed, allowed to dry slowly away from dust, and then varnished. (E. H. Jaques.)