APHENGESCOPE (Fr., Megascope, Aphenge scope ; Ger., Aphengeskop, Wunderkamera fur Undurchsichtige) In the aphengescope or opaque lantern, also sometimes called the megascope, the images are projected upon the screen by reflection instead of by transmitting the light through transpar encies. The first magic lantern of this nature appears to have been invented by Euler, the mathematician, and was described in his " Letters to a German Princess." In his letter to her of January 8, 1762, he gives diagram A, and says that he had the honour of presenting her with one of the lanterns six years previously.
The object to be optically projected was placed in the back of the lantern at B, and opposite it in a sliding tube in the front of the lantern was the projection lens A. It contained two side wings, with lamps and mirrors to illuminate the object. In the Metropolitane " is a plan of this or another lantern by Euler for the projection of opaque objects. Prior to Euler's invention it seems that the rays of the sun were used to illuminate an object the image of which was then thrown upon a screen. Really practical instruments, however, were not con structed until about the year 1839, when Mr. Longbottom used the oxyhydrogen light in The reflector is pierced for the lamp-chimney, and also for the object glass. The apparatus may be compared to a huge egg, having one end sliced off obliquely, against which opening the picture to be projected is placed. There is no condenser needed, and although the size of the projected picture is necessarily of limited dimen conjunction with opaque lanterns, with which he gave exhibitions at the old Polytechnic Institute, London. Twenty years later Mr. Chadburn, of Liverpool, obtained a patent for a lantern of the opaque class, in which he also used oxyhydrogen illumination.
In all aphengescopes great illuminating power is necessary if a large picture upon the screen is desired. Fair results may be secured by using an ordinary optical lantern for the purpose, arranged as shown in B, in which A is the objective lens, L the lantern containing the source of illumination, and B the space in which to place the picture or object to be projected. When very large pictures are needed two lanterns may be sions, it is very popular in the United States for projecting enlarged views of cartes, coloured lithographs, etc., of small size. Illustration E represents a more recent form of aphengescopic attachment. The objective lens is at A, the object or picture to be projected at B, and a mirror at M, the latter serving to divert the rays coming from the illuminant D from a horizontal into a more or less vertical direction. A biunial form of this apparatus is shown in illustration P. Two lanterns are used, and two mirrors M concentrate the light to a common point B, where the picture or object to be projected is situated. Naturally, a much brighter picture is the result, and accordingly a much larger image upon the screen is permissible. The aphengescope suggested by Mon. Trouve, and introduced by Mon. Molteni,
of Paris, was termed " l'Au,xanoscope," and the used, as indicated in illustration C. The double source of illumination makes the picture or object B very bright, and the lens A transmits a brilliant image to the screen.
A most ingenious form of aphengescope was invented some years ago by an American, Dr. Foote, of New York, who termed his instrument the " Polyopticon Wonder Camera." Great illumination is secured by the use of a concave mirror M, in illustration D, gathering up all the rays from a lamp at D, and reflecting on to the picture at B ; the objective lens is shown at A.
simplest form of this apparatus is provided with a lamp on each side of the objective. The incandescent electric lights are fixed in tubes, the ends of which are provided with reflectors, with a hole in each to allow of the insertion of the conducting wires. In the pattern having three illuminants, two are used as just described ; while a third, at the back, is utilised for trans mitting light direct when inserting an ordinary transparency in the groove provided.
Devices on the same principle have lately been used for projecting the image of the dial of a watch upon the ceiling so that a person lying in bed may switch on the lamp and see the time.
A practicable form of aphengescope for attach ment to optical lanterns is hexagonal in plan, and is made of either wood or metal. At the back are two doors permitting of one object being shown while another is being prepared. At the front are three holes, the central one having a flange to receive a lantern objective of long focus and large diameter, and the side holes being bushed with brass tubing to receive the draw-tubes of two lanterns. The object to be shown is placed in the aphengescope behind the objective.
Modern lighting facilities are responsible for the recent revival in opaque lanterns. Incan descent gas, incandescent electric lamps, and the more powerful electric arc, supply all that is necessary with regard to illumination in order to procure brilliancy of image upon the screen. But with improved lighting facilities comes the possible evil of overheating the subject, and consequently damaging the originals. One pre caution against this evil suggests itself in the form of an alum tank, interposed between illuminant and subject, which performs the function of absorbing heat rays without unduly interfering with the course of the illu minant rays. Such a device has often been used in conjunction with lanterns when projecting transparencies, especially when the slide is required to remain stationary for any length of time. Moving panoramic pictures for the aphengescope have been suggested, and in this case it may not be necessary to introduce an alum tank or heat absorber ; but for lecture purposes, when the picture is fixed for a con siderable period, an absorber is a useful adjunct.
"Mirroscope " is the name given to a recent form of aphengescope, and it is designed separately for electric, acetylene, and incandescent gas light.