We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 175
1. It may be in front of the object so that the light emitted
shines directly on it. This is the original device and gives the greatest amount of light (fig. 92-93); or the radiant may be tilted (fig. 105, in).
2. The second method is to have the light not in front, but a
mirror reflects the light from the radiant upon the opaque object (fig. 94, 95). This is usually a more convenient arrangement than the above, but a certain amount of the light (between 10% and 25%) is lost when reflected from a mirror.
§ 279. Use of a condenser or concave reflector with opaque projection. — This is frequently employed for the object is often at a considerable distance from the radiant, and too small a part of the light from the radiant would be available but for the help of the condenser.
In most cases only the first element of the condenser is used. This projects upon the object or the mirror a cylinder of parallel rays (fig. 90, 103) . Sometimes also a converging lens of long focus is put in the path of the parallel cylinder to concentrate it more or less, depending upon the size of the object to be shown. Instead of a condenser, there is sometimes used a reflector (fig. 95, 96) behind the radiant.
§ 280. Darkness of the projection room. — Owing to the difficulty of obtaining a sufficiently brilliant screen image it is necessary to have the projection room very dark.
COMBINATION LANTERN SLIDE AND OPAQUE PROJECTION § 281. Daylight and twilight vision. — Nearly all modern apparatus giving opaque projection also gives transparency projection with a slight change. These two kinds of projection are mutually antagonistic for the adjustments of the eyes of the spectators. For transparency projection the image is so brilliant that the eyes are adjusted for daylight vision in large part, while for the opaque projection the image is so dim that the eyes should be adjusted for twilight or night vision.