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 171
face instead of traversing it, — that is, it must extend in the opposite direction from that used with the transparency.
The light falling upon the face of the opaque object must then be reflected from each point. But unlike the tranteparent object, in which practically all of the light illuminating each point of the object goes directly to the projection objective (fig. 91), with the opaque object, each point reflects the light irregularly and in all directions within the entire hemisphere ( 1 80 degrees, fig. 90) . This being the case, only a part of the light reflected from each point can get into the projection objective, all the rest falling outside the objective. Of course, the larger and closer the objective, the more of the light will be received; hence, in selecting an objective for opaque projection, keep in mind that the greater the diameter of the lenses the more light from each point can be received, and consequently the more brilliant will be the screen picture.
It is assumed in this discussion, and in the accompanying diagrams (fig. 90-91), that the opaque object is black and white and that it and the transparent lantern slide are of the same size; that both are lighted by a similar beam of parallel light rays, and that none of the light is lost by absorption.
§ 274. Relative amount of light for the images with transparencies and opaque objects. — If, for example, as in the diagram, the projection objective can receive but 20 degrees of the hemisphere of light from each point, then 160 degrees will fall outside the objective and not aid at all in the formation of the screen image. If the objective could take in all of the light from each point, the opaque object would give as brilliant a screen image as the lantern slide, but the actual proportion of light represented by the angle of twenty degrees is only three per cent, of that represented by 180 degrees. As only three per cent, of the light from each point helps in the formation of the screen image of the opaque object, the opaque object can give a screen picture only three per cent, as bright as the transparency where practically all of the light helps to form the screen image (fig. 90-91).
In practice, how great a proportion of light serves for the screen image and how much is absorbed or lost depends upon the opacity