320 PROFESSOR L. BECKER ON THE DISTRIBUTION OF 



(100-88). It is of some importance to know the upper limit, if not the accurate 

 durations, of the exposure belonging to Photograph VI. On Photograph No. VII. 

 three images are eccentrically superposed (see fig. 6). The order of magnitude of the 

 time for which they were illuminated can be found from the degree of blackness of 



the background which belongs to each image. In this 

 way I find that circle (b) is due to a short exposure 

 of the order of a second, the faint circle (c) (dotted in 

 fig. 6) which is faintly visible within the corona is the 

 lunar disc during a very long exposure, and semi- 

 circle (a) must belong to an exposure of about 

 10 seconds. Now (b) is exactly at the position on 

 the plate, as determined by its distance from other 

 pictures, at which the plate-holder was locked by the 

 electromagnet of the propelling mechanism. The 

 contacts made by the clock for unlocking and locking 

 the plate-holder are between the contacts for exposures, 

 and hence no part of the exposure which produced (c) 



can have contributed to (b). As (c) belongs to the longest exposure, (b) must be due 

 to some portion of the 2'8-seconds exposure. The first part of this exposure must 

 have contributed to picture VI., because the plate moved (see the instantaneous 

 photographs of the protuberances on the lunar disc of picture VI. and in the coronas 

 of VI. and VII.) while an exposure was going on. The exposure given to No. VI. is 

 therefore 9'02 + 2'82 T = 1T84 T, and the combined exposure for abc of No. VII. is 

 100'88, less the exposure of No. VI. T may lie between O'l and 1 second (see above). 

 I should add that the lunar disc on Photograph VI. is exactly round and not blurred 

 in the least. Apart from the instantaneous pictures and trails of prominences on the 

 lunar disc, the picture VI. is perfect. 



(b) Measurement of Curve of Equal Blackness on Photograph VII. I shall now 

 investigate whether the three eccentrically superposed images on this photograph can 

 be utilised in this research. Let the curves of equal intensity of the corona be circles, 

 and let us consider the curves at some distance from the sun. Let C and A (fig. 6) 

 be the centres of two photographs of the sun (not moon, as it was supposed in No. Ga) 

 and MM' and NN' be circles of radius r along which the corona has an intensity i. 

 To centre C belongs the exposure ^ and to A the exposure t a . Along circle MM', i has 

 been exposed during t t and on this is superposed an exposure to i (di/dr) Ar during 

 t a , i.e., i has been exposed during t^ + t, and (di/dr) &r during t 2 . This additional 

 radiation has the same value with opposite signs at two opposite points of circle MM'. 

 In the same way along circle NN' intensity i has acted during ti + t 3 , and (di/dr) Ar 

 during t a . Hence the radiation i was exposed during t } + 1 2 along a curve which lies 

 between the two circles MM' and NN' and in such a way that the mean of the 

 distances (p and p') from C of two opposite points of this curve is equal to r. Along 



