The Optical Magic Lantern Journal (August 1889)

eon Tho Optical Magic Lantern Journal and Photographio Enlargor, 19 This combination gives excellent sharpness whatever be the nature of the light in the lantern, but unless the focus is rather long, the field is apt to be so round, as not to give equal sharpness at the sides as at the centre. This could, of course, Wp Eig cL be remedied, as it is in the photographic camera, by making use of a stop; but while in the camera it entails only a slight increase in the exposure, it is fatal in the lantern that is being used for exhibition purposes, on account of its diminishing the light too much. If the focus mus¢ be very short, then it is requisite that the back combination be of large diameter, and is not only possessed of considerable magnifying power, but also have a great amount, of negative aberration ; otherwise it will Fic. S. be impossible to geta flat field. The diameter of back lens must be greater than usual in order to take in all the cone of light from the condenser. The front lens need not be large, as the central portion only is called into requisition. By considerably lengthening the tube of the Petzval portrait objective, and substituting a combination of the form here shown for the back, we have an object glass which better than any other gives equal sharpness over the field. Figure 8, which represents Dallmeyer’s improvement on the Petzval system, explains an objective which, subject to a very slight modification, gives an image that shows scarcely any falling off at the margin. The modification, which applies only to our diagram, is the still further separation of the front from the back lens by merely lengthening the tube. Some of the cheap French portrait lenses, now so much used for the lantern, can often have their flatness of field improved by unscrewing the back combination and removing the ring by which these lenses are separated, and placing them in juxtaposition. This flattens the field by increasing the negative aberration of the lenses. In regard to the mounting of the object-glass, it is desirable, if not necessary, that in every instance it be fitted with rack and pinion, and that for convenience there be two nuts to the pinion, so as to be easily eperated from either side of the lantern. It is also necessary that facility be afforded for withdrawing the lens a considerable distance from the slide, so as to enable a lens of long focus to be used when necessary. Probably the best and most convenient mount is the telescopic one of Mr. Wrench, shown in Fic. 9. fig. 9, in which provision is made for object-glasses of either long or short focus. In this, sets of lenses mounted in tubes of similar size are inserted by merely being pushed into the outer draw tube of the telescopic front.