1871.] Intensity of Daylight in Eclipse 0/1870. 



513 



and from l h 40' up to the time of totality it was impossible to make any 

 observations, as the sun was never unclouded for more than a few seconds 

 at a time. As the illuminated portion of the solar disk gradually increased 

 after totality, the clouds rapidly disappeared, the amount falling from 

 9 (overcast =10) to 3 in about fifteen minutes. The observations were 

 then regularly continued to within a few minutes of last contact. 



Although the disk and by far the largest portion of the heavens were 

 completely obscured by clouds during totality, rendering any determina- 

 tion of the photochemical action perfectly valueless for our special object, 

 it was yet thought worth while to attempt to estimate the chemical inten- 

 sity of the feebly diffused light at this time, which certainly is capable of 

 producing photographic action. 



Immediately after the supposed commencement of totality the slit was 

 opened, and the sensitive paper exposed for ninety-five seconds. Not the 

 slightest action, however, could be detected on the paper, and we therefore 

 believe that we are correct in estimating the intensity of the chemically 

 active light present at certainly not more than 0*003 of the unit which we 

 adopt, and probably much less. 



The Table containing the experimental numbers and the graphic repre- 

 sentation of them are given in the memoir. By a graphical method the 

 relative areas of the sun uneclipsed at the times of observation were 

 obtained ; and these are seen in column 3 of Table II., the area of the 

 unobscured sun being taken as unity. 



Column 2 gives the results of the photochemical observations made 

 during the eclipse obtained from the graphical mean, and corrected for 

 variation in the sun's altitude, the total chemical action immediately before 

 first contact being taken as unity. Column 1 gives the apparent solar 

 times of observation. 





Table II. 





1. 



2. 



3. 



ll / 

 12 44 



0-915 



0961 



12 54 



0-876 



0-880 



1 16 



0-G8G 



0-637 



1 24 



0-555 



0-534 



2 2 



o-ooo 



o-ooo 



2 9 



0-165 



0-127 



2 25 



0-307 



0-338 



2 34 



0-4G4 



0-498 



2 44 



0-601 



0-602 



2 54 



0-725 



0-736 



3 4 



0-876 



861 



From these observations we conclude tliat the diminution in the total 

 chemical intensity of the sun's liyht during an eclipse is directly propor- 

 tional to the magnitude of the obscuration. 



vol. xix. 2 s 



