USEFUL TO THE MICROSCOPIST 1117 



It is often necessary in the examination of a photo-micrograph of 

 diatomic or other periodic structures to determine at what rate per inch 

 or per mm. the structure is in the original object, the amplification of the 

 photo-micrograph being known. 



Example : In a photo-micrograph of a diatom amplified 735 diains. 

 12 dots can be counted in -3 of an inch. At what rate per inch is the 

 structure in the diatom ? 



-, magnifying power x number counted . 



space counted 



735 x 12 = 29,400 per inch. 

 3 inch 



(2) If the answer is required in rate per mm., the space in which 

 the number is counted being in inches as before, then, because 1 inch 

 = 25*4 mm. 



735 JL 12 -. =1157-5 per mm. 

 3 inch x 25-4 



(3) Suppose a rule divided in mm. is used to determine the space in 

 which the number on the photo-micrograph is counted, and the rate per 

 inch is required ; if twelve dots can be counted in 7 mm., then, because 

 1 inch = 25 -4 mm. 



735 x 12 x 25-4 



7 mm. 



= 32,004 per inch. 



