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202 PREPARATION OF LANTERN SLIDES [Cn. VIII
image (fig. 6-8, 112). For a square form two marks are needed (fig. 13, 113).
§ 3 13. Actual size of the free opening with lantern slides. — The
sizes given above are the measurements from the extreme edges of the plates. The actual size of the picture to be projected is always less, as part of the slide is covered when inserted in the carrier. The mat between the slide and its cover, and the binding around the edge lessen the size a variable amount. It requires from 5 to 10 mm. all around the edge for the binding and the part covered by the slide-carrier. This leaves a clear opening in the lantern slide of that much less. The smaller the slide to start with the less will be the proportionate amount of clear space left after the mounting of the slide.
The free opening of the American slides is rarely greater than 70 x 75 mm. and much more frequently the free opening is considerably less.
§ 314. Diameter of the condenser required for different sized lantern slides. — The final element of the condenser next the lantern slide (fig. i, 2, 114) must be somewhat greater in diameter than the diagonal of the free opening of the lantern slide to be projected.
The accompanying figures show the British, French and American, and German standard sizes of lantern slides with the minimum diameter of the condenser which should be used with them (fig. 114).
§ 312a. There is some confusion as to the exact outside measurement of lantern slides. For example, the exact size of the British square slides is 3>4*3/4 inches (82.5x82.5 mm.) In the two French works consulted (Trutat, p. 311, and Fourtier, tomeii, p. 18) the British size is given as 80 x 80 mm.
In Italy the size is given as 85 x 85 mm. In the German work of Wimmer the exact size is given (82.5 x 82.5 mm.). Neuhauss speaks of slides 85 x 85 mm. (p. 27).
The standard French slides are given as 85 x 100 mm. This is one of the standard sizes in Germany and Italy. Hence, it is concluded that the standard British slide is meant whenever 80 x 80, 82.5 x 82.5, or 85. x 85. mm. slides are mentioned. Also that the standard French and American slide of 3^x4 inches (82.5 x 100 mm.) is meant whenever slides of 85 x 100 mm. are mentioned.