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CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 177
This apparatus is designed to project opaque objects as large as 22 centimeters in diameter, at a magnification of five to ten with a 30 ampere current. For a smaller object one may magnify as high as 25 diameters. With a 50 ampere current and a larger reflector the magnification may be from 14 up to 37 diameters.
In this instrument the carbons are horizontal and in the optic axis. The parabolic reflector (R) serves to direct the light in a parallel beam along the line of the optic axis.
It takes considerable time for the eyes to adjust themselves, hence, if one passes quickly from opaque projection to lantern slides the screen images are dazzling. On the other hand in passing from lantern-slide images to opaque images, the eyes being adjusted for daylight vision, the screen images seem exceedingly dim at first, although the screen image may be as brilliant as it is possible to obtain with the best apparatus. After the eyes gain their twilight vision the images on the screen appear much brighter, as if the light had been greatly increased. As old observers put it: "It is necessary to get the brilliant sunshine out of the eyes before the relatively dim screen images are satisfactory."
§ 282. Dim and brilliant light in combined projection. — This difficulty can be avoided in two ways :
1. In showing lantern slides, the current may be lessened until
the light forming the image of the transparency is of about the same intensity as is that of the opaque object with the full current.
2. A neutral tinted glass of the proper shade can be put in the
path of the beam going to the lantern slide, to tone down the brilliancy (§ 282a).
§ 282a. In 1908-1909 this difficulty was in part overcome by Mr. A. O. Potter by putting a tinted glass of the proper light reducing power in the path of the beam going to the lantern slide. This reduces the image of the transparency to the same dimness as the opaque object, hence one can pass from one to the other without any adjustment of the eyes.
If only lantern slides are to be shown, the tinted glass can be removed and the full light employed.
Some combined lanterns, as those of the Bausch & Lomb Optical Co., and perhaps others, are now regularly supplied with the light reducing glass for the magic lantern part.