Modern magic lanterns; a guide to the ... - Yesterday Image

THE OPTICAL SYSTEM. 65a 4| inch will be wanted. In getting a condenser, twopoints should have attention : the glasses should be mountedquite loosely in their brass rings so as to rattle when shaken,if not, when expanded with the heat of the lamp they willmost likely break, and the lenses themselves should be asthin as possible at their edges. The slide should be as nearto the condenser as the carrier will allow. For photographicenlarging, condensers will be required of a size dependentupon that of the negative from which the enlargement is tobe made ; for good illumination its diameter should be at leastan inch longer than the diagonal of the negative.The objective for projection purposes is, in the cheapestlanterns, what is called a plano-convex (a, Fig. 53), or ameniscus (b and c), but these are not at all to be desired.Fig. 53. Lantern Objectives.especially in those of short focus ; for anything over 10 inchesthey are frequently very satisfactory. With the introductionof the rapid modern dry-plate in photography, however, thenecessity for the very " quick " lenses for portraiture haslargely disappeared, and portrait lenses as used by photographersanswer all the requirements of the average lantemistadmirably, and are frequently to be met with at a very lowprice. This lens, shown diagrammatically in Fig. 53, d, wasyie invention of Professor Petzval; and it or its modificationsin the hands of Dalhneyer are excellent for projection. Severalopticians now make special lantern lenses, based more or lessupon this pattern, one or two of which we have used at onetime or another with satisfaction. This type not only allowsa large amount of light to pass, but it also possesses a veryflat field, a quality the nature of which must be explained.