274 Scientific Proceedings, Royal Dublin Society. 
correctly exposed and developed negative a difference of 3 per 
cent. in the intensity of light can be detected. It should there- 
fore be possible, by correct exposure, to detect the corona on 
the sky when the skylight forms 993 per cent. of the light, 
and the corona $ per cent. The problem is to obtain nega- 
tives showing this difference, and two diametrically opposite 
opinions were held as to the best method of securing this. The 
American observers and Comte de la Baume Pluvinel held that 
short exposures and slight photographic action were necessary, 
whereas most English authorities held that it was only by long 
exposure and great photographic action that we could hope to 
secure the faint extensions. 
The photographic action on a plate is the product of three 
factors—the effectiveness of the lens, the duration of the exposure, 
and the sensitiveness of the plate. The effectiveness of a lens or 
mirror was defined by the International Photographic Congress at 
2 
Paris as 100 , where @ is the aperture, and fthe focal length of 
the optical instrument; so that in comparing the photographic action 
on plates taken with various instruments, the formula becomes 
100 ea t. s.. Where ¢ equals the time of exposure, and s is the sensi- 
tiveness of the plate. Assuming plates of normal sensitiveness 
2 
used throughout, we get 100 5% as the formula for comparing 
the photographic actions on the various plates used for the corona ; 
and the application of this formula to the eclipse plates of 1898, 
April 15-16, gives very interesting results. 
With the English instruments exposures of 2, 5, 15, 20, 50, 
120, and 150 seconds were given; and applying the formula we 
get :— 
4-inch lens, with enlarger :— 4-inch lens, 60 inches focus :— 
1 second = 044. 1 second = ‘444. 
2 seconds= 088 2seconds= 888 
TR ve 8) Bi loge) ) it | 27228 
SS hyp yess de) = ee Oe Oy B51 f=) LOGGG 
A) IS 88 20 = EER eR 
BOG pop pe 25222 50 ,, = 22-292 
120) = 5838 190.) = 5esae 
150 ,, = 6°666 ° 150 ,, = 66°666 
