410 BELL SYSTEM TECHNICAL JOURNAL 



If the factors Bn and Af„ can be made approximately constant for 

 all the screens, the coefficients for either form of the Fourier Series are 

 directly proportional to the change in the amount of transmitted light 

 for specified pairs of positions of the cosine screens. 



Description of the Instrument 

 The process which the analyzer is required to carry out consists of 

 superimposing the function to be analyzed on a cosine screen and 

 measuring the variation in the transmitted light when the cosine 

 screen is moved along the x-axis. This is repeated with a different 

 cosine screen for each harmonic which it is desired to measure. 



A schematic diagram of the instrument is shown in Fig. 2. The film 

 containing /(x) is placed in a holder at A and strongly illuminated by 



LIGHT 

 SOURCE 



A 



Fig. 2 — Diagram of optical system. 



an incan4escent lamp and condensing lens. An enlarged image of 

 /(x) is formed at .B on a window bounded by two knife edges .750 

 inch apart. Functions of different length are accommodated by 

 adjusting the optical enlargement so that the image of the portion of 

 /(.r) to be analyzed will just fill the window. The cosine screens slide 

 in a track directly behind the window, and receive the image oi f{x). 

 The transmitted light is collected by another lens and brought to a 

 photocell. 



A series of cams and levers is arranged to bring the cosine screens 

 out of a drum shaped magazine in which they are stored into the 

 optical path, give them the small motion required for analysis, and 

 return them to the magazine, which is then rotated to bring the next 

 screen into position. These operations are all automatic, and the 

 attention of the operator is required only for the adjustment of the 

 enlargement and focus and resetting of the cams at the beginning of 

 each analysis. A photograph of the instrument is shown in Fig. 3. 



The variations in the photocell output take place at the rate of about 

 two cycles per second. These are recorded on a moving chart by an 

 instrument similar to a high speed level recorder,^ differing from it 

 chiefly in having a linear instead of a logarithmic scale. 



8 "A High Speed Level Recorder," VVente, Bedell and Swartzel, Jour. Acous. Soc. 

 Amer., vol. 6, p. 121, January 1935. 



