From this positive a "relief" is produced, in which these white spots will be raised ones, and can, therefore, be cut down before the tinfoil is applied. This process has the further advantage that the positive can be either direct (of the same size as the original negative) or copied in the camera of any size*; but it also shares with the Woodbury process the drawback of only being economical when considerable numbers are required.
Perfect lantern slides are realised from gelatine plates, either by direct contact printing or in the camera, combining not only perfect transparence, but every gradation of tone from the warmest chocolate to the coldest black.
The days of collodion for outdoor work are nearly gone, and for lantern slides they are certainly numbered. Their cold tone might be passed over, or, perhaps, improved; but their fatal defect is the difficulty in obtaining due transparence in the shadows. Gelatine plates for the amateur seem to present the greatest number of advantages of any process, and there is no reason why - for amateur work at any rate - a larger camera than 3 1/4 in. square should be needed. Either from the negative an enlarged print direct may be made, or from the positive (or lantern slide) an enlarged negative might be produced of sufficient sharpness for all practical purposes. With the ordinary lantern, the condensers, being 4 in. .in diameter, are large enough to cover the effective part of the 3 1/4 in. square picture, while, with a very little modification,* enlarged negatives quite equal to full-sized direct negatives might be produced.
It is often complained of enlargements that they lack crispness, and undoubtedly this is frequently the fact. But is it due to inherent defects in the process or defective manipulation? To commence: how many negatives are themselves sufficiently crisp to bear examination with the ordinary focussing eyepiece. If they will not bear this amount of amplification it is, of course, impossible to produce sharp enlargements from them. On the other hand, sharp photographs have been produced of minute objects, such as diatoms, in which the amplification has been carried to thousands of diameters. A magnification of upwards of 100 diameters, with a degree of sharpness still requiring the aid of a magnifier to distinguish details, is within the range of the common microscope objective; and, having repeated the experiment with different objectives and the same success, I have no hesitation in saying that there is no reason why negatives should not be satisfactorily enlarged to any extent. (G. Smith.)
It may be interesting to many to know the old system of working, so that they can realise the difficulties attendant upon it, which gave rise to so much prejudice against the albumen process, particularly amongst amateurs. Accordingly, before commencing the details of the modern method, we shall digress for a moment to give a brief outline of the old process, so that the more recent improvements may the better be understood, and therefore appreciated. In the earliest methods the plate was coated with plain un-lodised albumen, an 1 then dried. The iodising of the film was afterwards effected by exposing it to the vapour of iodine, in the same manner as in the daguerreotype process, until it became decidedly of a yellow tint. It was then sensitised by immersion for a short time in a solution of nitrate of silver strongly acidified with acetic acid, similar to that employed in the calo-type process. An improvement on this plan was effected by adding an iodide to the albumen itself, which simplified matters considerably, as it did away with at least one troublesome and disagreeable operation. The method of working was this: - A certain quantity of albumen was taken, and to it was added the proper proportion of iodide of potassium.
The addition of bromide was also made by some workers. The whole was then whisked into a stiff froth. The directions usually given for this part of the operations were that the albumen should be whisked until the vessel containing it could be inverted without the contents running but. After standing some hours for the albumen to subside it was ready for use.
The plates were coated in the following manner: - They were first affixed to a plate-holder, which consisted of a piece of guttapercha fastened on the end of a stick, as pneumatic plate-holders had not been invented at that period. The guttapercha was made sticky by melting it in the flame of a spirit lamp, and the plate attached. The albumen was now poured on and distributed, and afterwards equalised by centrifugal force, by giving the plates a rapid rotary motion by spinning the stick between the fingers. The plate was then detached, and put into the drying-box. This consisted of a box containing a number of grooves into which shelves of porous wood fitted horizontally, the box being mounted on levelling screws. Previously to using the box the shelves were removed, and placed, together with the box, either in the sun or in front of a file to thoroughly desiccate them, so that the wood should be rendered as absorbent as possible. The shelves were now replaced in the grooves, and the box carefully levelled.
It was then ready for the reception of the plates, which, it will be seen, were dried entirely by the moisture from them being absorbed by the wood of the shelves and box.
Now it is clear that if the box were not accurately levelled, or if the shelves had become warped, the films would be of unequal thickness, and the plates, consequently, useless. Up to this point dust was the greatest enemy, for the smallest particle settling on the film was almost certain to cause a spot, and it will be noticed that up to this time the plates were always in a position to favour floating particles coming into contact with them. The sensitising was effected in a bath of aceto-nitrate of silver in much the same manner as at present. But the development was materially different from that now practised, inasmuch as the picture was brought out with a saturated solution of gallic acid, with a few drops of acetonitrate of silver added, instead of, as now, with pyro-gallic acid. The time occupied in the development of a picture by gallic acid was rarely much less than 1/2 hour, and it frequently took as much as 2 hours if it were at all under-exposed. But, by the method about to be described, it does not take much longer to develop and fix an albumen than it does a gelatine plate.