(2) It is required to calculate the distance (from a screen of given size) of a lantern fitted with a lens of given focal length so that the projected image may suitably cover the screen. The picture width of the lantern slide being 3 in. (as above) and the screen measuring, say, 71. ft. (90 in.), we get the ratio of 9013 = 30. The lens having a focal length of 6 in., it must be at a distance from the screen of 31 times its focal length, that is, i5 ft.
(3) It is required to calculate the size of the screen to be suitably covered by a cinema film projection (picture portion of film positive about 17/25 X 23/25 in.) at a distance of, say, 331 ft., with a lens of 5 in. The proportion is obtained by dividing the distance from lens to screen by the focal length, and by decreasing the quotient by I unit, this giving 79. The dimensions of the screen will therefore be— Width . . . 79 X 21/25 73 in.
Height . . . 79 X 17/25 54 in.
A chart (nomograph) of the kind shown in Fig. 11)9 enables these problems to be solved at sight (L. P. Clerc, 1921). Fig. 199 is worked out for the two sizes (T6 mm. and 9.5 mm.) of sub standard cine film, and consists of three scales, viz. : (I) focal lengths of projection lens from .1- in. to r in. ; (2) distances of projector from screen from 1.1 ft. to 35 ft., and (3) sizes of picture from ift. to Io ft.
By laying a straight-edge, e.g. a perfectly straight ruler, across the chart so that the edge falls on marks, on any two scales, the position of the ruler on the third scale indicates the dimen sion required.
For example, to find the distance from the screen for a 4 ft. picture with i6 mm. film, using in. projection lens, the required distance is 9 ft. With 9-5 mm. film, it is Ill ft.
Again, to find the focal length required for a 6 ft. picture at 8 ft. from screen when using i6 mm. film, the required focal length is o°58 in.
8o8. Communication Between Lecturer and Lanternist. In a well-conducted lantern show there should be no occasion for the lecturer to address the lanternist. The latter should be told by signal when to begin showing the slides, when to change the picture, or when to interrupt projection. In a hall used regularly for lantern lectures it is best to arrange two bell pushes and three wires for working two small lamps clearly visible to the lanternist. A white light, for instance, for showing a slide, and a red light for interrupting the projection. An electric bell, arranged to strike once only, or, better, a " buzzer," the sound of which is more smothered, may be used, one stroke for showing a slide and two for an interval.
For travelling use, there is the " lecturer's lamp," a dark lantern for reading notes by, which is fitted (on the side facing the lanternist) with a red window covered by a shutter except when the latter is raised by pressing a lever. Another device, acting by sound, is a clapper.' 809. Centring the Light Source. The adjust ment of the position of the light for the uniform illumination of the screen is done as described in § 759 for regulating the light in an enlarging lantern with directed light.
This adjustment should be made before the public are admitted, the lamp being then turned off. The lens should then be capped either with a rotating cover or a metal cap, and should not be uncovered until after the light is on and the first slide is in place ready for projection.
81o. Arrangement of the Slides. The slides should be cleaned and arranged in their proper order before the lecture. Nothing is so disagree able as the projection of finger-marks or the appearance of a picture other than the one announced. Care must be taken, when arrang ing the slides, to place all the same way, as indicated by the spots, these being placed uppermost, so as to be readily seen at the moment when they are taken out of the box by the lanternist.
Avoid having to ask again for a slide which has already been through the lantern, as this always leads to the showing of several slides before the desired one is found, there being nothing to single it out. Subjects required twice in a lecture should be made in duplicate, or, if not, the slide should be bound with white binding strips and distinctly marked, a corre sponding mark being made on a card or cover glass placed later on in the series.
When slides have been kept in a very cold room or have been carried in very cold weather, they are liable to become covered with a dew when exposed to the air of the hall. It is there fore well to warm them by putting them in racks placed close to a heating 8ii. Placing the Slides in the Carrier. As previously stated, the image can be projected on to an opaque or translucent screen.
In both cases the slide must be placed in the carrier so that the lower part of the picture is at the top, i.e. the indicator label in French and American slides must be at the top, while the spots on English slides must be at the bottom. These marks must be turned away from the lens when the screen is an opaque one, but they must be turned towards the lens when showing through a screen.
Before the lecture starts, the lanternist should satisfy himself that the slides handed to him bear the spots 2 and that they are all placed the same way, Failing numbers or other indication, he should ascertain from the lecturer which is the first slide to be shown. The slides must then be placed at the side of or below the lantern, placed on edge in a long box, if possible facing the same way as they will be in the slide carrier, so that it will be unnecessary to turn them round before placing in the carrier. If it is not possible to replace them after use in the same box without risk of confusion, another box must be provided in which they must be put as each one is taken out of the lantern. A careful lanternist will return the set of slides entrusted to him in such a condition that they can be shown again without any need for re-arrange ment and even cleaning, the slides being never touched with the fingers except in the parts covered by the masks.
After the last slide has been shown, the lamp must be put out, but as far as possible it is necessary to avoid opening the lamp-house while it is still very hot, or at least it must be closed again immediately, so as not to produce draughts of air liable to cause cracking of the condenser. Some lanternists lit a shutter of asbestos inside the lamp-house. This is closed from outside by a lever, and protects the con denser before the doors of the lamp-house are opened. The lanternist should then cover the whole of the lantern with an old rug, for the purpose of causing it to cool slowly.