THE BOOK OF THE LANTERN. 95 



frame, it should be strong ; for if the sheet is tightly- 

 stretched, as it ought to be, there will be a very heavy 

 strain upon its support. Perhaps the best wood for the 

 purpose is bamboo, which combines the qualities of 

 extreme lightness with great strength. I believe that 

 bamboo frames can now be obtained commercially. 



The rule for finding the correct distance between lantern 

 and sheet is to add one to the number of times enlargement 

 required, and multiply by the equivalent focus of the lens 

 used. If, for instance, a slide measures 3 inches, and it is 

 desired to cover a screen 10 feet across, the scale of 

 enlargement is 40 times : and 41 times the equivalent 

 focus of the lens gives the required distance between lens 

 and screen. 



In order to find the equivalent focus of a lens, it is con- 

 venient, in the absence of special apparatus, to proceed as 

 follows : Focus upon a white surface an image of the 

 Sun or other distant object, taking care to place the lens 

 axis as nearly as possible in line with the object, and 

 perpendicular to the surface receiving the image. When 

 the image is sharply focussed, measure carefully the dis- 

 tance between it and the nearest surface of the lens. 

 Repeat this operation with the lens reversed, and measure 

 the distance to the same surface as before ; in this case, 

 the surface furthest away. The average of the two 

 measurements thus obtained is approximately the 

 equivalent focus of the lens. 



I am indebted to Mr, Taylor, of Leicester, for the fol- 

 lowing useful table : 



