THE LANTERN LECTURE 8o5. The Lecture Hall. When there is occa sion to build or to adapt a hall for lantern work it is preferable to select a hall which is much longer than it is wide, thus enabling advantage to be taken of the metallized screens. It is obvious that the size of the screen must be in accordance with the length of the hall, but the first rows of the audience must, as far as possible, be far enough from the screen to prevent it being viewed at an angle of more than (i.e. at a distance from the screen about equal to its width). If the floor of the hall is level, the lower edge of the screen must be at a greater height than is necessary with a hall built like an amphitheatre,' more especially so in a long hall. If the seats are fixed, they should be staggered from row to row, thus enabling each spectator to see the screen between the heads of the two people seated in front of him.
Unless it is quite impossible, the position of the lantern should be such that the optical axis of the lens cuts the screen in its centre ; other wise it is necessary to de-centre the lens on the lantern (§ In many lantern halls the screen is surrounded by a wide matt-black border, the drawbacks of which outweigh the advantages. The ideal is to surround the screen with a grey tint illumin ated so that its brightness is about n/Ioth that of the high-lights of the projected image and three times that of its deepest 8o6. The Illumination of the Hall. It is customary to put out all the lights while slides are being shown, except for sonic low-power lamps covered with coloured filters to indicate the exits. It is, however, possible to retain an illumination sufficiently bright to permit of note-taking after the eyes have become accus tomed to it. For this purpose only indirect lighting from the walls and ceiling must he employed, the sources of light and their diffusers being hidden from the eye. A slightly yellow light (lamps with amber screens or dipped in amber-coloured varnish) is generally best. Except for the lantern screen, no surface exposed to the sight of the audience must have a bright ness greater than no candle-metres (the desks of the orchestra, if there be one, must be shielded from view, since they must be much more brightly lighted). The illumination of a hori
zontal surface at the level of the spectators may be as much as x candle-metre in the front rows, then gradually rising to 2 candle-metre in the back rows. These limits may be exceeded if the lantern screen is at the rear of a stage and is thus protected from the major portion of the or dimmed except when the first picture is about to be shown.
807. Optics of Projection—Choice of Focal. Length—Size of the Projected Picture. The fol lowing three problems may arise in lantern practice— (I) It is required to find the focal length of light scattered by the ceiling and walls. The illumination of the entrance passages should be regulated so that the spectator is brought im perceptibly from the light outside to the dim light of the hall (L. A. Jones, 1920).
In the most usual case where the lighting of the hall is of either full brightness or practically nil, it is advisable that the switches 1 should be controlled by the lanternist, so as to ensure that the lights in the hall shall not be put out lens covering suitably a screen of given size, the projector being at a given distance (the distance reckoned from the lens). If, for exam ple, the picture width of the lantern slide is a maximum of 3 in. and if the width of the screen is to ft. (120 in.), the ratio between the image on the screen and that on the slide is equal to 120/3 40. Now, it is known that to enlarge an image 40 times (§ 62), the sharp image is formed at a distance from the lens equal to 41 times the focal length. If, therefore, tie distance from the lens to the screen is 26 ft., the focal length of the lens for complete covering of the screen will be 26 X 12 ± 41 in. 7-6 in. As a rule, it is unlikely that a lens with the exact focal length required will be available, and to be certain that the projected image will not tend beyond the screen, a lens will be chosen with the next greater focal length, e.g. one of 8 in.