The first and third method work well, without seriously affecting the simplicity of the apparatus. When the ground glass is fixed, however, in the back of the case, it is not in focus for the plate, and it is necessary to make an adjustment after focussing. This is not a serious objection, however, as every amateur will soon learn to mark his focus for infinite distance, at which mark 9/10 of all his outdoor pictures must be taken. (Prof. G. W. Hough.)
In computing the weight of the various items for a photographic tour, the glass almost invariably comes out at the head of the list, and the farther or longer the journey, so much more does the weight of the plates stand out preeminent; indeed, if one goes out on a trip with only 3 doz. half-plates, the glass will probably weigh nearly as much as camera, backs, and tripod, in spite of the stipulation with the maker to supply plates on " thin glass."
Next in importance to glass as a support comes paper, and it is quite easy to understand that the tourist in out-of-the-way parts might be able to take an apparatus containing a roll of sensitive paper, when it would be altogether impracticable for him to take an equivalent surface of coated glass, and in such a case the roller slide becomes of especial value.
The roller slide of Melhuish is tolerably well known, and is, we believe, now obtainable as an article of commerce. The slide is fitted up with 2 rollers, and the sensitive sheets are gummed together, making one long band, the ends of which are gummed to pieces of paper always kept on the rollers. The sensitive sheets are wound off the left or reserve roller on to the right or exposed roller, until all are exposed.
The rollers are supported on springs, to render their motion equal; they are turned by milled heads, and clamped when each fresh sheet is brought into position by nuts; a board is pressed forward by springs so as to hold the sheet to be exposed, and keep it smooth against the plate of glass; when the sheet has been exposed, the board is drawn back from the glass in order to release the exposed sheet, and allow it to be rolled on the exposed roller; the board is kept back while this is being done by turning a square rod half round, so that the angles of the square will not pass back through the square opening until again turned opposite to it; by opening doors the operator can see (through the yellow glass) to adjust the position of the sensitive sheets when changing them.
The remarkable similarity of such a slide to an automatic printing frame will strike the reader; and, like the printing frame, it possesses the advantage of speed in working - no small consideration to the photographer in a distant, and possibly hostile, country.
Fine paper, well sized with an insoluble size and coated with a sensitive emulsion, is, we believe, the very best material to use in the roller slide; and such a paper might be made in long lengths at a very low price, a coating machine similar to that constructed for use in making carbon tissue being employed. We have used such paper with success, and hope that some manufacturer will introduce it into commerce before long. But the question suggests itself, how are the paper negatives to be rendered transparent, and how is the grain of the paper to be obliterated? Simply by pressure, as extremely heavy rolling will render such paper almost as transparent as glass, a fact abundantly demonstrated by Woodbury in his experiments on the Photo-Filigrane process, and confirmed by some trials which we have made.
It must be confessed that roller slide experiments which we have made with sensitive films supported on gelatine sheets, or on such composite sheets as the alternate rubber and collodion pellicle of Warnerke, have been hardly satisfactory - possibly, however, from our own want of skill; while no form of the Calotype process which we have tried has proved so satisfactory as gelatino-bromide paper. (Photo. News.)
With ordinary photographic apparatus, it is necessary to begin by focussing upon the object that one desires to photograph, and to afterwards remove the ground glass and substitute for it a frame containing the sensitised plate. After this, the latter is uncovered and the cap is taken from the objective. It goes without saying that if the object to be photographed were an animal in motion, it would have been out of sight long before everything was arranged to take its image upon the sensitised plate. This difficulty has been surmounted in 3 ways. One consists in the use of objectives that give an equal sharpness with objects situated in very different planes, and that do not require focussing. The second way of getting round the difficulty is that adopted by Muybridge and Marey, and consists in setting up the apparatus in advance, focussing it upon a certain point, and then causing the animal to pass before it.
The third method is one whose principle was made known some time ago, and which consists in using a very portable apparatus formed of 2 symmetrical and interdependent chambers, with a pair of identical objectives, one of which serves for focussing and the other for obtaining the image.
The majority of the apparatus of this kind that have hitherto been invented give exceedingly small images, that are of not much practical account.
Prof. Hermann Fol has invented an apparatus on this principle which is a great improvement upon all other portable devices of the kind in use, and which he terms a "photographic repeating gun." This is placed against the shoulder like an ordinary rifle, and is thus given sufficient stability to make the images very sharp. The image, although it utilises only the central part of what the objective is capable of covering, measures 3 1/2 by 5 in. The apparatus contains 11 plates, which may be successively exposed within short intervals, without any other manipulation than that of raising the shutter and inclining the whole thing alternately in one direction and the other. Finally, it folds up in such a way as to make it convenient for carriage.