The magic lantern has long been a favourite instrument with children, and under past conditions, when the pictures thrown by it were hand painted, and were little better than rough daubs; it was only fit for the amusement of children. But now that photography is able to furnish pictures full of the most exquisite detail, pictures which are actually improved by being greatly magnified, the lantern takes a far more important place, and is at once raised to the position of a scientific apparatus of the greatest value. With the improved pictures too has come an improved instrument (see page 136). The old oil lamp, with its disagreeable smell, and its weak light, has given place to a lamp with three or four wicks burning mineral oil. The lenses too are now made on scientific principles, and for a small sum we can purchase a lantern fit for exhibition purposes. We may look forward to the time when every household will regard a lantern as a necessity, and even now they are by no means uncommon. Without question every amateur photographer should possess one. During the long winter evenings when other photographic operations are impossible, he can print from his negatives slides for the lantern, which with ordinary precautions will compare favourably with any that he can purchase at shops. We will now give plain directions by which this branch of photographic work can be readily accomplished.
For the production of slides we shall require some gelatine plates measuring 3 ¼ X 3¼ inches. This is the standard size of all lantern pictures, and no other size should be thought of. The plates ordinarily used for negatives will answer the purpose, and if the size cannot readily be obtained, quarter plate6 can be used. These measure 4¼ X 3¼ inches, so that an inch must be cut off either before the plate is used, or after it is finished. An American glass cutter can be used for this, but a diamond is, of course, better. Having chosen some suitable negatives, which may be on quarter plates, or even 5 X 4 plates, place one in a printing frame, with the film side up, just as if you were about to print a proof on albumenized paper. But instead of paper place above the negative one or the square plates just mentioned. By holding this against the negative, and holding the latter close to the red lamp, it is easy to choose that portion of the subject which it is most desirable to reproduce as a transparency, the negative being dimly seen through the plate. Now carefully replace on the table, and fasten in the back of the frame in the usual way. All is now ready for exposure. Daylight being far too intense for the purpose, we must use some kind or artificial light, and the most convenient is gas. If possible, have a table gas lamp with a good batswing burner. Light this before commencing operations, and turn it down to " the blue." (A capital burner is now made on purpose for photographers. It cannot be turned quite out, and is always ready when it is wanted.) Now hold the printing frame so that the negative faces the burner, at a distance of about 18 inches from it. Turn up the light for three seconds, and immediately turn it down again. This exposure will be about sufficient for a good negative, and using a gelatine plate of ordinary rapidity. If the negative be thin, place it three feet away from the light, and increase the exposure four fold. If, on the contrary, the negative be very dense, it may be placed much closer to the light; as in other branches of photography, the exact exposure can only be learnt by experience. At the same time it is all important that correct exposure should be given for each negative employed.'
There are several methods by which transparencies may be developed, indeed any developer can be used, if we arr not particular as to colour. The ordinary pyrogallic and ammonia developer will give a picture of a disagreeable yellow tinge, and although this tint can be partly removed by a clearing solution of citric acid and alum, the colour remaining is not satisfactory when the picture is seen on the screen. If pyrogallic developing is used at all, it should be employed in the form of the Beach developer already described. This gives a good black colour. Almost as good a result can be promised by using the soda developer. But this mode of development is specially liable to extreme yellowness, particularly if the operation be prolonged. It must therefore be followed by using the clearing solution containing iron, which has been already given in the chapter on development. This solution should be freshly mixed for the purpose.
The developer which in the writer's hands has given the best results for transparency work, is a modification of the ferrous oxalate method. Mix the developer as already recommended, by adding the powdered iron crystals to the oxalate of potash solution, and then adding a few drops of bromide of potash solution. So far we have the ferrous oxalate developer pure and simple, which may be used as it is. Here comes the modification. Mix as a stock solution the following: -
Citric Acid ... ... 5 ounces.
Distilled, or Rain Water... 20 ounces. Liquor Ammonia (88o)... 2 ounces.
The citric acid crystals may conveniently be rubbed down in a mortar. Add the water to them, and after transferring the mixture to a strong bottle, add the ammonia. This last addition will cause the evolution of so much heat that the crystals left unpowdered will quickly dissolve. If the solution be kept beyond a few weeks, a kind of mouldy fungoid growth will be observed in it, This can be prevented by adding to the solution when first mixed, a pinch of salicylic acid in powder. Doubtless, a drop of carbolic acid or a few cloves would answer the same purpose. This solution may be labelled "citrate of ammonia." The modified developer is made by mixing the solution of citrate of ammonia with the ferrous oxalate developer in equal parts.