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28 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I
3. In the slide-changer of the Spencer Lens Co.'s magic lanterns (Delineascopes) , the slide is laid flat, with the face up, i. e., so it will be toward the condenser when ready for projection. The edge which is to be uppermost in the ordinary vertical carrier, is toward the screen. Now when this slide-changer is used it turns the slide up in the vertical position so that it is in precisely the same position as with the ordinary slide carrier.
§ 36. Possible ways of inserting American lantern slides in the slide carrier. — The standard American lantern slide is oblong (10x8.2 cm.; 4x3^ in.), and the carriers are constructed to receive them lengthwise. While they would never be inserted with the short edge up, they can be inserted with either long edge up, and facing in either direction. This gives four possible positions in the carrier, only one of which is correct. That is, there are three wrong ways of inserting the slide in the carrier with the corresponding wrong images on the screen. It is not very uncommon for an audience to see all possible images of the same slide, and occasionally the wrong ones repeated once or twice. This is as inexcusable as it is unnecessary (fig. 10-12).
§ 37. Possible ways of inserting the square English lantern
slides.— These slides are 8.3 x 8.3 cm. (3^x3^ in.), and being square they may be put into the carrier with any of the four edges up, and of course with either face toward the lamp. This gives eight possible ways of insertion, seven of which are wrong. Square slides must have two "spots," (see fig. 13).
§ 38. Focusing the image on the screen. — When the lantern slide is in the correct position before the condenser (fig. 1-2) the objective must be at such a distance from the slide that the screen image will be sharp, and show clearly the printed matter and all the details of the picture. With the usual magic lantern the objective is nearly in the right position all of the time. But for any necessary final focusing there is a rack and pinion on the objective, or it is mounted in a tube with spiral movement. By turning the milled head of the pinion, or by turning the objective