THE BOOK OF THE LANTERN. 229 



will be sixteen minutes. To make the diagram more 

 explicit, the vertical squares 1, 2, 3, and 4, have been so 

 subdivided that the number of spaces in each indicates 

 the number of units of exposure, be that unit a second, a 

 minute, or an hour. The same rule holds good for en- 

 larging operations. Thus, supposing that we are working 

 with an optical lantern, and that the necessary exposure 

 at 1 foot from the lens is half a minute ; at 2 feet the 

 time will be two minutes ; at 3 feet four minutes and 

 a half ; and so on. The practical worker will have this 

 little bit of theory in his mind whenever he is operating, 

 and he will soon be convinced that the theory is strictly 

 correct. 



Another help in enlarging, which will be found useful, 

 is a little piece of apparatus, if it can be dignified 

 by that name, which I have lately made, and which I 

 call an exposing-gauge. It is so simple in construction 

 that any one can make it out of a couple of strips of card- 

 board. The arrangement is shown in fig. 57. The size 

 of the gauge is immaterial, but a length of 20 inches 

 will be found convenient. A slip of card of that length, 

 and about 1 inch in breadth, is cut with pointed ends, 

 each point having a hole pricked in it as shown. By these 

 holes, and with the assistance of a couple of drawing-pins, 

 the contrivance can be readily attached to any flat surface 

 upon which the enlarged image from the lantern is 

 focussed. Placed above this slip is another piece of card 

 slightly shorter, and with a round hole in the centre. The 

 two strips are bound together with pieces of tape glued 

 over their upper and lower edges, the two ends being left 



