AERIAL SCREEN (Pr., Ecran d'air ; Ger., Windschirm) A form of screen for giving relief and other effects for optical lantern pictures, its special object being to arrest the light coming from a lantern and to reflect it back to the point at which the projected image is to be observed. The " Bruce " aerial screen consists of a white lath, turning on a vertical axis in a plane parallel with the lantern lens. The mechanism and the rotating lath occupy a position in front of a black velvet screen or background, which absorbs all rays of light not falling on the lath and thus prevents them from reaching the eye of the observer. An ordinary optical lantern projects the subjects, which are preferably pictures of statuary in which fine photographic quality is present, giving as much rotundity as possible, and having a black background. The revolving lath, which takes the place of the ordinary lan tern sheet, is caused to rotate at a moderate speed, calculated to make one revolution within the time needed to satisfy the laws of persist ence of vision. Viewing it in broad daylight as it is rotated at the specified speed, it would present the appearance of a transparent cylinder. When the apartment is darkened and the pic ture is projected upon the rotating lath, the subject presents a somewhat solid aspect, and the audience will not be conscious of a revolving device, the illusion being caused by light from all parts of the image impinging upon the lath as it arrives at each and all of its positions. In virtue of the law of persistence of vision, there fore, a complete image will be made up. The chief object of this form of aerial screen is to bring about relief, but naturally a full stereo scopic effect is not obtained in this way. Another form of aerial screen consists of a column of vapour rising from the ground and acting as a reflector of the projected rays of light, just as a cloud in the sky may reflect the rays of the sun, but as the medium cannot be controlled as regards its reflecting surface it is only useful for producing weird effects in which absolute definition is not a necessity.
Yet another and much more recent form of aerial screen is that in which the laws of partial reflection are made use of ; it is termed " the invisible screen," because the medium upon which the image is actually received is hidden from the observer's view by the front of the proscenium. A method of reflection somewhat similar to that used in the old illusion known as " Pepper's Ghost " is adopted, but there are variations in the arrangement of the apparatus which make the results far more perfect and realistic. An observer situated in the auditorium at A (see the illustration) looks towards the pro scenium B, and sees in a dear plate glass C the aerial image D E, and at the same time observes at r any actors (real persons) who may be performing. The light rays constituting the spectre D E are arrested before reaching the glass C by a semi-transparent screen G. In the basement under the stage an ordinary optical lantern, or a kinematograph projector iat, is set up. A mirror j, placed at an angle of 45 deg. in relation to the optical axis of the lantern objective, diverts the light from its horizontal course into a vertical direction, so that the defined picture or image is received upon the semi-transparent screen G. In virtue of the angle of reflection being equal to the angle of incidence, whatever may be projected on C will be seen by the observer at A, and will appear to be situated in mid-air in the vicinity of F. Hence, the so-called " invisible screen " may be regarded as the aerial screen, although the aeriograph is seen in quite a different place. The chief object of the invisible screen in this case is to afford means for producing aerial images or spectra in combination with real actors ; whilst by the use at H of a kinematographic apparatus instead of an ordinary lantern many startling effects, otherwise impossible, are pro duced.