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 con- 

 siderably 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 Ameri- 

 can, 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 men- 

 tioned. 



