THE BOOK OF THE LANTERN. 29 



be found on page 95, bat in practice I find it a convenient 

 thing to use a tape measure. Upon the circular case of 

 this measure I have certain figures to the effect that such 

 and such a lens requires a distance of so many feet to give 

 a 15 or 18 foot disc. The place of the sheet having been 

 decided upon, it is then easy enough to pay out the 

 required amount of tape, and to fix the position of the 

 lantern. 



