The Optical Magic Lantern Journal (August 1889)

18 The Optical Magic Lantern Journal and Photographic Enlarger. The Magic Lantern: Its Construction, I!fumination, Optics and Uses. CuapTer TI.—Tue Opsecr Grass. WHILE on the condenser depends the equality of the illumination of the disc, it is the object-glass that determines the nature of the definition of the projected picture. Equality of sharpness is of primary importance. It is very unpleasant to see the centre of the disc very sharp, accompanied with a rapid falling off towards the edges. Much better were it that there should be a lowering of the definition in the centre, with a corresponding increase towards the sides. In this matter let us learn a lesson from the drop scene of a theatre. When stepped up to and examined from a distance of two or three feet, there is no real sharpness anywhere to be seen, and yet to the spectators at a distance it is quite sharp and uniform. This ought to be the effect produced on the lantern screen. No part should be so absolutely sharp as to challenge a comparison between it and other less defined portions. We are not now speaking of projections intended for being enlarged photographically, in which the sharpness should be of the very highest order obtainable, but of those that are to be viewed on the screen from a moderate distance. A good form of simple lens for a lantern objective is a one and three-quarter inch planoconvex lens of about six inches focus, with its flat side next the screen, and a three-quarter inch stop placed two inches in front. But an improvement on this consists in the form with which all the best lanterns were fitted before achromatic lenses were used for this purpose. In this the aberrations were diminished by employing two lenses, a plano-convex and a meniscus, both placed close together, convex surfaces next to the picture, and the meniscus next the stop. With an object glass of this form, the tocus was shortened to such an extent as to permit the lantern to be placed at a distance from the screen equalling the diameter of the disc. With lenses of such short focus, a square object, such as a picture of architecture, was shewn with a slight distortion of what is known as the fix cushion form, to obviate which many of the handpainted slides of the highest class were drawn with a slight barrel-shape distortion, by which they were projected upon the screen with perfect rectilinearity. Some very experienced lantern exhibitors prefer and use object-glasses of a similar character, but those are now composed of one, two, or even three plano-convex achromatics, screwed in cells fitting one into the other like the triplet system of French microscope objectives. In addition to giving a disc of great uniformity, this system possesses the advantage of allowing the exhibitor to place his lantern at any convenient standpoint, and to use or combine any of the lenses to fill his screen ; for with three lenses, each of a focus different from the other, we have in effect seven objectglasses, all of different foci. It is to be noted that with object-glasses of the class to which we have hitherto alluded, the smaller the flame the greater will be the sharpness of the image on the screen. Dr. Monckhoven has given it as his opinion that when a light no larger than Fie. 6. that of the limeball is used, the object-glass that will prove to be the best will be a single triple lens of a form similar to that shown in the cut, which is one of Dallmeyer’s (fig. 6). The object-glass that is at the present time made use of more extensively than any other is the portrait combination of Petzval. It consists of a plano-convex or slight meniscus achromatic convex at the outer end of a tube, with a separated pair of lenses, convex and concave, at that end next to the slide (fig. 7).