BRIGHTNESS OF THE CORONA. 129 



INTEGRATING PHOTOGRAPHS. 



Two members of the Association, the Rev. John M. Bacon 

 and Mr. F. Gare arranged for the exposure of photographic 

 plates to the general light of the corona during the eclipse, in 

 order to get a measure of its total photographic radiation. 



Mr. Bacon exposed an Ilford ordinary plate, quarter-plate size, 

 one half of it on the night of 1897, November 9, at 8 h 15 m> p.m., 

 to the light of the full moon at Coldash, near Newbury, and 

 the other half to the light of the corona during the eclipse 

 at Buxar; the exposures being for 80 seconds, and the moon 

 at the same altitude in both instances. 



Mr. Gare's plate was a half-plate Lumiere orthochromatic, 

 and was placed beneath a sensitometer which he had prepared, 

 divided into thirty squares, one thickness of tissue paper being 

 used in the first square, two in the second, and so on to thirty 

 thicknesses in the thirtieth square. 



This plate was exposed by Mr. E. W. Johnson, at Buxar ; and 

 Mr. Johnson is seen at his work in the photograph on p. 90. 

 A precisely similar plate was exposed under the same sensito- 

 meter by Mr. Gare and Mr. A. H. Johnston, at Dulwich, to the 

 full moon on 1898, April 6, at 10 p.m., the exposure in each case 

 being for 1 minute. The two plates were developed at the same 

 time and under identical conditions by Mr. A. H. Johnston. 

 Six plates from the same batch were exposed under the same 

 sensitometer to the flame of an amyl-acetate lamp at Faraday 

 House, with the kind assistance of Mr. Albert Campbell, by 

 Mr. and Mrs. Walter Maunder ; and the density of the deposits 

 on portions of these plates and of that taken by Mr. Bacon were 

 measured by Mr. Walter Maunder with the photometer of the 

 Imperial Dry Plate Company, with the kind permission and 

 assistance of Dr. J. J. and Mrs. Acworth. 



The results of the comparison, when reduced to an altitude of 

 38 for both moon and corona, the moonlight being also reduced 

 in both cases to the mean distances of the earth from both sun 

 and moon and the amyl-acetate lamp to a distance of 1 metiv. 

 are as follows for Mr. Bacon's and Mr. Gare's plates respectively. 



Mr. Bacon's plate. Mr. Gare's plate. 



Corona 8- 9 lamp 4*9 lamp 



Moon 3-3 0-7 



Corona = 2*7 moon 7'0 moon. 



The difference between the results given by the two plates is 

 no doubt chiefly due to their widely different colour sensitive- 

 ness, but partly also to the weather conditions on April 6. 



9 



