The Optical Magic Lantern Journal (March 1890)

76 Tho Optical Magio Lantern Journal and Photographio Enlarger. Lantern Slides and Lantern Slide Making. (Continued from page 61.) HOLDING the frame towards a screened kerosene or gas-light about 18in. distant,an average exposure of ten seconds is given by quickly removing and replacing the screen. A thin negative will need but two seconds; a very dense negative will require from twenty to thirty seconds, and one having a yellow tinge may take a minute. By making a few trial exposures and -developing them, the different capabilities of different negatives may be quickly learned, sothat one may tell ‘by instinct how long the exposure should be. For contact work many prefer to use 4 by 5 negatives, ‘selecting the centre for the picture ; but a better size is the 3} by 42 plate, which has margin enough to allow tthe proper locating ofthe sensitive plate and makes a Picture better proportioned to the lantern size. Instead of artificial light. diffused daylight or magnesium flash or ribbon light may be employed. By the contact method slides may be madc very rapidly during the evening, when one has leisure time. ~ Reduction by the camera is largely preferred, because the variations in the size of the image to be impressed upon the sensitive plate are so easily and quickly effected. Usually the camera is placed on a long board, holding at one end a frame, in which are removable kits or frames to hold different sized negatives. A printing frame with the back removed may be utilised for this purpose. The negative is best illuminated by facing that portion of the sky opposite to the position of the sun, though it is possible to use sunlight by interposing a ground glass between the window and the negative, and giving a shorter exposure. The space between the negative frame and camera should be covered by a light black cloth to exclude extraneous light. I prefer to have a special frame fitted tothe back of the camera, with a 3} by 4} aperture cut in the centre, arranged to support or holda 34 by 4 double plate-holder, rather than to employ a kit in a large holder, because it is more convenient and more easily handled. I generally employ a rapid rectilinear lens, though it is not necessary, as a wide angle lens will answer just as well. Any lens that will give a sharp picture stopped down to F-16 will do. Much study has been given to the matter of artificially illuminating negatives for reduction in the camera. The cheapest method is the magnesium ribbon light moved around in front of the negative, having a diffusing screen interposed. The use of an arc electric light diffused by a screen of ground glass has been suggested, the light to be above and in front of the negative about four feet distant; it thus strikes the negative at an angle, and is said to illuminate it equally. There have been three favourite kinds of developers for lantern slides, pyro, hydroquinone, and ferrous oxalate. The two latter do not stain the fingers. Pyro should be preserved with sulphite of soda, and combined with such alkalies as soda and potash. Four ounces of sulphite should be mixed with one’ ounce of pyro. This is undoubtedly the quickest developer, and, when the exposure is nearly correct, imparts a brownish tone to the slide. Not more than two plates should be developed in succession in one solution. A working formula is as follows :— Pyro vee ise < 2 grains. Sulphite of soda aes ster TM 35 Carbonate of potash ... a cae Water 1 ounce. If the shadows’ develop too dense, the amount of pyro should be decreased. A hydroquinone developer contains more hydroquinone in proportion than there is pyro in the pyro developer to get the’same amount of density ; but the difference is made up by the greater number of plates that can be developed with it. The following is used successfully :— : Sodium of sulphite H ydroquinone 1S 55 Water . sea --» TI ounce. To this is added fifteen grains of carbonate of potash, which forms the developer with which from six to eight plates may be developed in succession. (To be continued ) Hardwich on Scott’s Warm-Air Saturator. 40 grains. SOME Short time since Mr. A. W. Scott, of Weston super-Mare, sent me one of his ‘‘ warm-air saturators,”” with a request that I would try it and report the result. The saturator was charged with “ gasoline,” and three pints of the same fluid were sent for renewal. I have experimented carefully with this saturator, and am much pleased with its performance. It is equal, and possibly superior, to Broughton’s vapour tank, which is saying a great deal, as this tank, in my opinion, is nearly perfect in working, and if it had been incapable of explosion could not have been superseded. Ives’ porous saturator does not charge the oxygen so highly with ether, and is consequently more liable to ‘“‘snaps ” and “ passing back”’ towards the end of the lecture; but the experiments of the late Mr. Barber, conducted under my direction, proved that it is safe from fear of dangerous explosion. Mr. Sutton, of the firm of Cutts and Sutton, who was present, can testify that when it was purposely filled with explosive gas, and the flame drawn through it so completely that it set fireto the bag on the other side, the only effect was to produce a report about as loud as asmall pistol. Nothing gave way or burst, even when the experiment was several times repeated. Whether Mr. Scott’s saturator is as safe as lves’, I am not able tosay. He mentions, ina private letter to me, that there are three feet of air passages inside it, but he does not say of what diameter, consequently Ido not know whether there would be any chance of what is called “ short circuiting” from possible imperfection in the manufacture. This was where the Broughton tank broke down. There were seven feet of — tortuous windings, but the solder gave way on a particular occasion, and the oxygen in consequence passed by an easy route, emerging from the H pipe in an imperfectly saturated and explosive state. Such a thing could not happen in an_ Ives’ saturator, because the barrels are full of flannel, and the air tubes go straight from end to end.