BLUE-VIOLET LIGHT IN THE SOLAR CORONA ON AUGUST 30, 1905. 311 



2. The Photoijraphs (Plate I ). 



The pendulum was started about a second after Mr. H. MAYOR, who watched the 

 contact, gave the signal that totality had l>egun. Before the last exposure was 

 finished sunlight apju-.-in-d, hut I shut it oft' by stepping in front of the object-glass, 

 and it was about 3 seconds before the shutter automatically closed. That is to say, 

 totality lasted about 1+207 3 = 205 seconds, as compared with the calculated time 

 of 210 seconds. Accordingly, the tenth exposure lasted about 43 seconds. 



The plates (two halves of a whole plate) were developed together in the same tray 

 by a strong developer (Imperial standard) for 7 minutes. I developed at the open 

 window at star-light, keeping the plates covered most of the time. The photographs 

 show a great deal of contrast, and this has proved an advantage in measuring them. 

 The background of the long-exposed negatives is dense, due to the brightness of the 

 sky. This diffused light, whose intensity I had underrated in the design of the 

 apparatus, produced an impression even for the shortest exposures, darkening a square 

 on the plate equal in size to the opening in the plate-holder. I find, from measure- 

 ments, that the intensity of the diffused light equals that of the corona at a distance 

 of I'l solar diameter from the sun's limb, i.e. 0'6 in unit of the intensity of the corona 

 at a point 1 solar diameter distant from the limb as found from the formula 7. 



In the preliminary report I have said that the plate-holder failed to move in the 

 designed manner, due to some parts of the apparatus having been damaged in 

 transit. Owing to this accident, there is a multiplication of images in the sixth 

 and seventh pictures, and not only the first five photographs are an inch (2 solar 

 diameters) apart, but also the next two, which were meant to be at twice that 

 distance. In consequence, the successive exposures to the diffused light overlap on 

 the plate, with the effect that on one half of each of Photographs I. to VII. there is 

 the same duration of exposure to the diffused light as on the adjoining half of the 

 neighbouring photograph. This has enabled me to separate the intensity of the 

 corona from that of the sky. There is no overlapping on Photographs VIII. and IX., 

 and though they did not, on that account, furnish data for the intensity formula, they 

 supplied H series of equal-intensity curves of the corona, which are required for the 

 reduction of the other pictures. Fig. 3 explains the conditions. 



i n m B v 



IX 



Fig. 3. 



The photographs are numbered I., II., &c., and the exposures (see 1), 1, 2, <kc. 

 The lines at the top give the extent of the area illuminated at each exposure by the 



