3G 



Dr. H. H. Turner. On the 



" On the Brightness of the Corona of January 22, 1898. Pre- 

 liminary Note." By H. H. Turner, D.Sc., F.R.S., Savilian 

 Professor. Eeceived January 18, — Read February 7, 1901. 



1. In a former note* I gave some account of measures of brightness 

 made on photographs of the corona of 1893 by Abney's method. The 

 same method has been used on the coronal photographs taken in 1898 

 and in 1900 (in 1896 none were obtained owing to cloud), and a large- 

 number of measures have been made, though the work is not yet 

 complete. Pending the completion and publication of this work, it 

 seems advisable to publish the present note, as one or two results 

 have been arrived at which may be useful to others in the forthcoming 

 eclipse. 



2. As regards the method of measurement, sufficient has been said 

 (for the present purpose) in the paper already quoted. It need only 

 be added that in place of the revolving sectors a graduated wedge of 

 gelatine was used to diminish the comparison beam, according to Sir W. 

 Abney's more recent methods. The wedge or sectors are mere inter- 

 mediaries between the coronal image and the standard squares, and no 

 considerations beyond those of convenience are involved. The wedge 

 is much more convenient, and the work can be done with it twice as 

 rapidly. 



3. But a new method has been adopted of representing the results,, 

 which, though an elementary change in some respects, has had the 

 important consequence of suggesting a more satisfactory law for the- 

 variation of coronal brightness with distance from the sun. The only 

 simple law (so far as I am aware) which has hitherto been formulated 

 was that proposed by Professor Harkness in 1878, viz. : — 



Brightness a (distance from sun's limb) -2 . 



Visual measures made by Thorpe and Abney in 1886 and 1893 could 

 nob be reconciled with this law ; though I showed in the paper already 

 quoted that if the distance be measured from a point within the limit 

 (about I radius within), the law approximately satisfied the photo- 

 graphic measures. 



I have now been led to a completely new law, viz. : — 



Brightness oc (distance from sun's centre)* 6 , 



which, though still on trial, is supported by a fair amount of evidence,, 

 and the suggestion arose in the following way : — 



4. The brightness curve in the previous paper was obtained by 

 plotting brightness against distance. This gives a curve of hyperbolic 



* < Roj. Soc Proc.,' vol. 66, p. 403. 



