THE CORONAVISER 253 



about the axis of the scanner it is turned more slowly about its own 

 axis. A hardened pin Q attached to the arm of the lens mounting rides 

 on a cam which is fixed to this gear and thus imparts a cyclic tilting 

 motion to the lens. There are another similar gear and cam (not 

 shown) mounted opposite this one which serve only to compensate 

 for the weight of the working gear so that the rotating mechanism 

 may be balanced to a high degree of precision. 



The lens behind the scanning hole forms a stationary enlarged image 

 of the scanning hole on the photosensitive surface of the photoelectric 

 cell. 



The reproduction of the image is done entirely by electrical means. 

 At the opposite end of the motor a C-shaped permanent magnet ro- 

 tates about a cylindrical iron core around which there is wound a coil 

 of wire. The electric current thus generated in the coil is smoothed 

 into a true sine wave by tuning the circuit of the coil so that it resonates 

 at the frequency of circular scan which is approximately 30 cycles per 

 second. The resulting wave is split into two components 90° apart 

 in phase and these, after amplification, are impressed on the deflector 

 plates of the reproducing cathode ray tube so that the spot may move 

 in a circular path. 



At the generator end of the motor there is also a reduction gear box, 

 the slow speed shaft of which turns once for a complete cycle of the 

 radial scanning motion, i.e. once a second. This shaft is geared to 

 that of the potentiometer unit B so that its sliding contact also re- 

 volves at this rate. The potentiometer winding is continuous around 

 the circle and connections are made at the opposite ends of a diameter. 

 The scanning voltage from the generator circuit is fed into this unit 

 and as the arm revolves the amplitude of the sine wave is made to 

 vary uniformly between its minimum and maximum values. Thus 

 the scanning spot spirals outward, scanning a complete image of the 

 field, and then inward, giving another complete image, so that there 

 are two images per cycle of the radial scanning motion. The spot 

 does not follow the identical path on the two scans; the outward scan 

 crosses over the lines of the inward scan along one diameter and inter- 

 laces along the diameter at 90 degrees to this. The radial resolution 

 along the latter diameter is therefore double that along the former. 

 Since the frequency of the radial component of the scanning motion 

 is approximately one cycle per second, these two resolutions are 15 

 and 30 lines respectively. 



The glare of a clear sky is uniform around the solar image and 

 therefore as the scanning spot travels around the field it gives rise 

 mainly to a direct current in the photo-cell. The coronal features. 



