AN OPTICAL HARMONIC ANALYZER 



411 



The present instrument has been designed to take records of f{x) 

 which are from one-sixteenth to five-sixteenths of an inch long and no 

 higher than their length. The focal length of the enlarging lens is 

 1.5 inches. The collecting lens is placed quite close to the cosine 

 screens, and forms an image of the enlarging lens on the plate of the 

 photocell. With this arrangement the patterns of both f(x) and the 

 cosine screens are well diffused on the photocell plate, so that surface 

 variations in sensitivity of the plate are unimportant. The illumina- 

 tion is uniform across the field to ±2 per cent. 



Fig. 3 — The optical harmonic analyzer. 



The cosine screens were made on photographic plates, by printing 

 from variable density motion picture sound track negatives containing 

 records of pure single frequencies. The pattern thus produced is 

 about 1X2 inches. The increase in width of the track from about 

 one-tenth of an inch in the negative to one inch on the plate was 

 secured by making a "contact" print with negative and plate slightly 

 separated, and moving the plate sideways under the negative w^hile 

 printing. 



The important requirements for the screens are good wave form. 



