Optical projection: a treatise on the use of the lantern in exhibition and scientific demonstration (1906)

SCREENS AND LANTERN ACCESSORIES 131 lacing is required, but it may be advisable to tie the ends of the bottom roller by pieces of twine to the floor. The ring at one end of the top roller should always be tied with twine to the line, to prevent the screen slipping about in raising or lowering. Care is necessary in lowering, to let the two sides down alike, a third person in the middle rolling up the screen tightly as it descends. If the screen has to go on a wall, three nails will be all that is required. A water-tight wrap of oil-cloth or macintosh should always be provided for a screen of any kind, unless a sheet packs in the lantern-box. Portable frames for erecting screens can be procured of all opticians, but are most suitable for the smaller sizes (say 10 feet and under) in private rooms. A common form is shown in fig. 70, the poles being built of three short pieces connected by tubular sockets on the ends, so that the stand for a twelve-feet screen will pack into a case about 4^ feet long by six inches square. These stands are to be had fitted with pulley-rollers at each corner, when they are equally suitable for either a sheet or a paper- faced screen; the latter being simply hung, while a sheet is first strained over the pulleys, and then laced to the poles by twine or tapes. FlQ . 70 Another simple plan for elevating a roller screen, is to provide four pieces of pine-wood, each 7 feet or more long, and about 3x1 inches in section. Quarter- inch holes are bored every three inches for a couple of feet from each end, and four bolts with flange nuts are provided. Then two of the pieces can be clamped together by two of the bolts, so as to make one prop, and these props can either be simply stood on end and secured by guy-ropes, or may be fitted into base-pieces, or may be furnished with buttress pieces at the E2