228 THE BOOK OF THE LANTERN. 



be smaller or larger. But with regard to exposure he 

 worked entirely by rule of thumb, or rather, I might 

 sa y> by no rule at all. It was all guesswork, and, 

 although he tried many pilot slips of paper with watch in 

 hand, he failed to turn out any really correctly-exposed 

 pictures. His failure was chiefly due to his utter ignorance 

 of the law in optics, which has been already considered 

 on page 118. 



Referring once more to fig. 39 on that page, let the four 

 squares numbered 1, 2, 3, and 4, be printing-frames 

 placed at distances of 1, 2, 3, and 4 feet from a 

 candle-flame. Let us suppose, also, that we have ascer- 

 tained by experiment that the plate or paper in the first 

 position (No. 1) is sufficiently affected by the light if it 

 remain there for one minute. (This is, of course, merely 

 stated as a case in point. Bromide paper at such a distance 

 would be sufficiently exposed, under a normal negative, in 

 about eight seconds, while a chloride plate under such con- 

 ditions would want two minutes or more.) Then, if we re- 

 move the frame to position No. 2 at 2 feet from the light 

 source the necessary exposure will not be doubled, as some 

 might think, but quadrupled. For the square of 2 is that 

 number multiplied by itself, i.e., 4. The right exposure, 

 therefore, will be four minutes. Removing the frame to 

 position 3, we must once more square that number in 

 order to arrive at the right number of minutes, for ex- 

 posure at this increased distance. 3 x 3 = 9. Therefore 

 nine minutes will be the time. It is easy to see that when 

 the printing-frame is removed to the farthest distance of 

 all, which is 4 feet from the light source, the exposure 



