118 BULLETIN OF THE 



parts, a series of negatives is absolutely necessary, commencing 

 with very short exposures, and ending with very long ones. 



The process of obtaining drawings of the negative was as fol- 

 lows : First, an enlarged positive was made. Upon this is laid a 

 transparent gelatine film, and the whole is placed in a strong light. 

 The details of the image were scratched upon the gelatine film with 

 a needle-point. The film being removed lead pencil dust is rubbed 

 into the engraving, and an impression made upon paper. This 

 picture is a reverse of the proper outlines, but a second gelatine 

 tracing restores everything to the normal order. 



The measurement of the brightness of the corona is surrounded 

 with difiiculties, drawings from photographs are untrustworthy. 

 Comparison of original negatives would be better, but this is liable 

 to much uncertainty. If, however, we have for any eclipse a series 

 of photographs, and also a determination of the total light by 

 means of a Bunsen photometer, data will have been provided for 

 an approximate comparison with future eclipses* An application 

 of this process to the present eclipse showed that the intensity of 

 the coronal light varied inversely with the square of the distance 

 from the sun's limb. Prof. Harkness' conclusions respecting the 

 total light of the corona of July 29th, 1878, are as follows : 



1. The total light of the coroaa was 0.072 of that of a standard 

 candle at one foot distance ; or 3.8 times that of the full moon ; or 

 0.0000069 times that of the sun. 



2. The photographs show that the coronal light varied inversely 

 as the square of the distance from the sun's limb. 



3. The brightness of every part of the corona is given quite 

 approximately in terms of the brightness of the full moon by the 

 expression 



B = ^ (23' + 100' cos d) 



in which h = distance from the sun's limb in minutes of arc and = 

 the latitude of a point measured from the sun's equator. For very 

 small values of h this formula fails. Probably the brightest part 

 of the corona was about 15 times brighter than the surface of the 

 full moon or 37000 times fainter than the surface of the sun. 



4. The corona of December 22d, 1870, seems to have been 7i 

 times brighter than that of July 29th, 1878. 



