On the Brightness of the Corona, of April 16, 1893. 407 

 Table III. 



d = distance 

 from limb in 

 solar radii. 



Brightness compared with moon. 



Calculated 

 005/ + 0-182). 



Obserred visual. 



Obseryed 

 photographic. 



1886. 



1893. 



1893. 











(0-40) 



(1-54) 



0-1 







0-49 



0-64 



0-2 







0-33 



0-35 



0-4 







0-18 



0-15 



0-6 



0-047 



-0*43 



090 



0-083 



1-0 



0-038 



0-034 



0-026 



0-036 



1-4 



0-031 



027 



0-017 



0-020 



1-8 



0-024 



0-022 



0012 



0-013 



2-2 



0-019 



0-017 



0-0104 



0-0088 



2-6 



0-015 



-013 



-0092 



-0064 



No measures of brightness were made visually within 0-6 of a 

 radius of the sun from the limb. It would be interesting to have this 

 comparison made. For the region where we have a comparison, it 

 ^ippears that the light falls off photographically more rapidly than 

 visually- This is in accordance with experience, the faint extensions 

 having been seen more easily than photographed. 



As regards the central portions, we have some indirect information ; 

 for the total brightness of the 1893 corona was found to be equivalent 

 to 0'026 Siemens' unit at one foot or 0-022 candle, i.e.,, rather more 

 than the value assumed above for the moon. 



Integrating numerically for the part in the annulus extending from 

 €'6 radii to 2-6 radii, we find that this portion is equal to 0-20 moon 

 photographically and 0-25 moon visually. This leaves about 0-75 

 moon (visual) for the part within this distance (i.e., from the limb to 

 €•6 radius), while photographically the value got from the curve is~ only 

 0-44 moon. It seems as though the corona were altogether brighter 

 visually than photographically, in the ratio of about 3 to 2 ; but this 

 conclusion needs confirmation. 



An attempt has been made (in the column " calculated " of the 

 above table) to represent the brightness by a formula. The American 

 photographs of 1878 suggested that the coronal light varied inversely 

 as the square of the distance from the sun's limb. Abney and Thorpe 

 find that this law does not hold ; but the photographic observations 

 <5an be made to obey the law approximately. The calculated numbers 

 are obtained from the formula 



0-05/((^ + 0-18)2, 



2 H 2 



