254 ,BELL SYSTEM TECHNICAL JOURNAL 



however, that is the streamers, arches, etc., give rise to alternating 

 components and only these components are amplified, the direct cur- 

 rent being eliminated by resistance capacity coupling of the cell to the 

 amplifier. Inaccuracies of alignment and any non-uniformity of the 

 intensity of the glare across the field give rise to strong low-frequency 

 components and because of the high glare levels that were occasioned 

 by the location and by the siderostat mirror it was found necessary 

 to filter these components out of the amplified currents. A high-pass 

 filter is inserted between the first and second stages of the amplifier 

 for this purpose. A low-pass filter with a cut-off at 3750 cycles is also 

 included to cut out the noise at frequencies above the desired band. 

 The top frequency is 3600 cycles for the 0.04 inch diameter scanning 

 hole that was generally used. After amplification the signal current 

 is made to modulate the intensity of the cathode-ray beam. 



The electrical frequency spectrum of the television image consists 

 of the fundamental scan frequency (about 30 cycles) plus a large 

 number of its harmonics. By varying the characteristics of the high- 

 pass filter it was possible to eliminate the fundamental and several of 

 the lower harmonics as desired. This became advantageous in study- 

 ing the prominences and smaller coronal details which give rise almost 

 entirely to higher harmonics and are reproducible therefrom with a 

 good degree of accuracy. In addition, for these smaller details, the 

 coupling capacity between the first and second stages of the amplifier 

 was greatly reduced so that the gain at the upper end of the band was 

 considerably enhanced in relation to that at the lower end. 



The light of the corona is practically identical with that of the sun 

 in its spectral characteristics and a caesium sulphide photo-cell 

 which has a maximum sensitivity in the green was used. See Fig. 3. 

 It was found that by using gas amplification in the photo-cell adequate 

 sensitivity was obtained. The inner corona has a surface brightness 

 of about the same magnitude as the full moon and the sensitivity of 

 the apparatus was checked by obtaining images of the moon in its 

 various phases. 



It is convenient to measure light intensity levels in millionths of the 

 brightness of the sun's surface. The brightness of the full moon is 

 about 2 millionths and it is known that the inner corona falls off fairly 

 rapidly with distance from a brightness of a little more than one mil- 

 lionth measured within one minute of the solar limb. The level of 

 the glare was measured from time to time by means of a photronic 

 cell behind an aperture placed at the focus of the objective. On days 

 when the haze in the sky was very noticeable but yet not in the form of 

 clouds its brightness at 2 minutes from the limb was as high as 6000 



