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.
30 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I
images are desired. When, however, the screen distance is 10 meters (30 ft.) or over, it is not usually necessary to focus for each slide.
If the screen distance is very great (20 meters; 65 ft. or more) the operator cannot tell by his eye alone when the screen image is perfectly sharp. In such a case he must have an assistant stand near the screen to tell him when the image is sharp, or he can use good opera-glasses and determine for himself.
When the focus is once found for these long distances it is well to mark in some way the exact position of the objective ; then in future the operator can be sure of good screen images in the same position provided the lantern has not been moved.
§ 39. Hints on running the lantern for a demonstration lecture.
—It frequently happens that in a demonstration lecture, slides are to be shown at several different times. Ordinarily the arc lamp is turned out during the intervals ; but to make sure that the desired slide can be shown without delay, the arc lamp can be left burning all the time, and to avoid lighting the screen a mask can be put in front of the objective (fig. 14). A "push-through" carrier (fig. 6) should be used, and the next slide to be shown put in one of the compartments. The other compartment is left vacant, and this empty compartment is put in front of the condenser. If the slide were left in position all the time it might become over heated and break.
Whenever the slide is called for it is pushed into position and the mask turned aside. This will bring the picture on the screen almost instantly.
A mask or shield for the objective is much more important for the slow starting lights like the Nernst, than for the arc (§ 146, 169, 202, 217).
§ 40. Collecting and arranging the lantern slides at the close of an exhibition. — After the exhibition is over be sure to remove the last lantern slide from the slide-carrier. It not infrequently happens that the last slide is left in the carrier, and the lecturer's set is thus rendered incomplete.