The Optical Magic Lantern Journal (April 1894)

68 The Optical Magic Lantern Journal and Photographic Enlarger. slides. For absolute certainty in working, the slides should have no shake whatever in their frames. The mode of procedure is the same whether a set is of two or half-a-dozen; but for the sake of brevity, we willconsider the former. It must be borne in mind that under no circumstances must the adjustment of the stops and runners be interfered with so long as they render coincident the two template slides. Now place slide No.1 in the lantern and observe if the picture stands fair and square: if it does, then it is best left alone; it must of course be seen that the slide is close up to the stop at side of lantern. Then place the other slide in its lantern, and observe whether the two pictures coincide; the probability will be that they do not. If the second picture is too low, then the requisite amount must be planed off the frame so as to bring the picture lower down on the stage. Care must be exercised in the operation of planing so that too much is not taken off, for although in a set of two slides only, this can be rectified by taking a shaving off the other, this means of rectifying would hardly do in a set of six slides. The end of the frame that touches the stop must of course be treated in a similar manner. After a slide has been registered, it is well to again try the lantern by means of the templates before putting the slides away as correct. The process of registering the slides will be found both slow and tedious, but this is well repaid at the time of exhibition, when one, having ascertained that his lantern is in register, has merely to place his slides in as far as they will go, knowing that when the light is turned up the slide will be projected on the screen in correct register, and will require no dodging about to get it in right position. It will be observed that in Fig. 1 there isa dotted line a little distance above the runner; this represents a strip of wood which Mr. Wood informs us he always carries with bis lantern outfit in case he may have occasion to use any of the old small slides which are not infrequently to be met with. 203 The Royal Institution, Unique EXPERIMENTS MODIFIED FOR OPTICAL PROJECTION. Proressor Dewar has stated that in his earlier experiments with liquid air there were great difficulties in exhibiting them to a large auditory by projection, for hoar frost and ice quickly condensed upon the outsides of the glass containing vessels, interfering with the projec tion of the phenomena taking place inside them. Fig. 1. The accompanying cut, Fig. 1, shows how, at his last lecture, he made tbe solidification of common air visible to all in the theatre. K isa large tube, and W a smaller one, both at the outset nearly full of liquid common air. Both tubes are double ones, or “jacketed,” with a highly perfect vacuum between the two skins, to almost arrest the entrance of beat from the air of the theatre. The interiors of the vessels, by means of the tubes TT, were placed in communication with two powerful machine air-pumps driven by a gas engine, down in one of the laboratories. Thus the liquid air in both, by its own rapid evaporation, cooled itself still lower until at last, at the bottom of the tube W, asolid button of common air was formed, looking like ice. ; An ordinary high vacuum in the jacketing space would have been of no use for the purpose. The jacket is first filled with heated mercury, free from air bubbles, and then the metal is made to form a Torricellian vacuum in the parts represented in the cut, after which the glass tube is sealed at A, leaving nothing-but mercury vapour in the space above. Liquid air in a tuft of cotton wool is then painted over the outside of the bulb N, to bring down the trace of mercury vapour as a mirror inside N, then the tube is sealed with heat at R, and N is detached. The dotted portion of the cut is far too short to be at all in proportion to the rest of the diagram, for the sake of saving space, but it answers to explain the principle of the construction of these vessels. Fie 9 Fig. 2 shows how Professor Dewar is able, by the aid of the lantern, to show the liquefaction by cold of the air of the theatre, in an open tube, consequently at the norma] pressure, K is the open tube with its vacuum jacket, W is a teat tube coming to a blunt point at its lower end; this tube, at the outset, is nearly filied with liquid oxygen. By means of the tube N a powerful air pump is attached, and the cold produced by the rapid evaporation of the liquid oxygen causes the outside air to condense on the outside of the tube W, to trickle down its sides, and to fall in