The Optical Magic Lantern Journal (March 1900)

32 The Optical Magic Lantern Journal and Photographic Enlarger. the lens is not. working at { but» something considerably less. In the case of reducing from a half-plate negative so that its length would be just included on a lantern plate, the nominal stop £ would be really equivalent to one of only ;4. This, then, is about the maximum intensity that can be obtained, and it may happen, especially on a dull day, that the illumination thus afforded is by no means sufficient for some negatives. Again, another advantage of making slides by contact is that negatives of a small size only are required. Thus the weight to be carried, and the cost of materials and apparatus, are both reduced, and even where these considerations are of no importance, the possibility of using shorter focused lenses to embrace the same angle is a manifest advantage; for with the same relative stop the short focused lens has much greater depth of focus, hence ifa 3 inch lens instead of a 6 inch lens can be used owing to the small size of the plats on which the negative is being taken, larger stops can be employed and shorter exposures given. The smallest sized camera generally employed’ for negative making, even by those whose aim it is to produce lantern slides by contact, is a quarter-plate, although I believe that some photographers use a 31 inch square camera. It is manifest that the whole area of a quarterplate cannot be reproduced on the slide, and unless a square-shaped picture is to be shown on the screen some parts even of the 34 inch square negative cannot be utilised. I measure the opening of the rectangular masks that T find most generally useful. It is 2.9 inches long and 2 inches wide, that is to say, it is a little less than the size of the plate that the camera which I wish to see on ‘the market would take. This sized negative then would be amply sufficient for the production of slides by contact. Some precautions would have to be taken in printing to get the best results; a card 41 by 31 inches with a central aperture of 31 by 28, on one face of which a mask with aperture 3 by 2.1 should be pasted could be laid in a quarter-plate printing frame furnished with a plain glass, the side on which the mask is, being put'next to the glass. In the aperture of the card the negative could be placed, and on this the lantern plate could be laid and would be held firmly in place if across the back of the card two strips of thin card were pasted leaving a space of 34 inches between them. ‘The back of-the printing frame being closed in thé usual way, the exposure could be made without any danger of fogging from light getting round the edges of the small negative to the sensitive film behind it. I recommend a quarter of a half-plate in preference to the half of a quarter-plate, because I consider the proportions better, as if necessary a mask with a slightly wider opening than that indicated above might be used. The small camera should be capable of racking in to 2 inches and extending to about 8 inches, or if possible rather more. Two doublets of 2 inch and 3 inch foeus would probably be sufficient for use, as if these were anastigmatic lenses they would cover the plate sharply to the edge with an aperture of ~ or i, and on account of their short focus would even at these apertures possess a considerable depth of focus, and where longer focused lenses were necessary, the front combination could be removed and the back one used as a single lens with focal lengths of 4 and 6 inches respectively ; the additional 2 inches to which the camera would extend would allow of objects near the camera being photographed, even when the 6 inch single © was being used. The double backs for this camera might be very simple in construction. There would be no need for flexible shutters, as the area of the shutter when drawn would be so small, that it would neither be in the way nor be liable to catch the wind and set up vibration in the camera. The shutters therefore might be made of metal and the backs thin, so that a dozen carrying in all 24 plates would occupy but little room. In fact the whole kit—camera, two lenses, exposure shutter, and twelve double dark slides—would occupy but little space and would not weigh many pounds. One thing connected with the camera, however, should not be reduced in size and thatis the tripod. It does _ not follow that a large camera requires a taller tripod than a small one; as a general rule the stand should be sufficient to raise the camera about 5 feet or 5 feet 6 inches from the ground, so that the position of the lens may be at about the same height as the eye of a standing spectator of the view. Of course, somewhat slighter legs would suffice for the little camera as the weight that they would have to support would be less, but the height must not be reduced, Probably if such cameras were put on the market by camera makers there would be a large demand for them, and plate makers would cut plates to the size required for them ; but if such plates could not be procured,. it