SUMMARY OF RESULTS AND SUGGESTIONS. 163 



of an exceedingly brilliant light. To increase the equivalent 

 exposure, therefore, and to do nothing else, is simply to secure 

 the fogging of the plate before the development can bring up 

 the faint lights. The third element is the amount of atmos- 

 pheric glare present in the immediate neighbourhood of the 

 faint extensions. With a large amount of such glare, or if it 

 diminishes in brightness with distance from the sun more 

 slowly than does the corona, it is clear that a limit will soon be 

 reached -beyond which these rays cannot be photographed. 

 Otherwise the problem is to give an exposure sufficient to show 

 the ray, but not long enough to bring up the general glare, and 

 to use plates so prepared that the intense action set up by the 

 inner corona shall not spread far enough to interfere with the 

 outer. 



The evidence before us does not enable us to decide whether 

 the limit of exposure in a fine eclipse has yet been reached. It 

 would certainly appear to be a duty to attempt on the next 

 occasion to increase the exposures given this time as far as 

 possible ; although it is perfectly likely that the result of such 

 an experiment will only be to prove that the limit was nearly 

 reached on the present occasion. 



There is, however, some ground for hope that if favoured with 

 skies as clear as we had in India, the present record exposures 

 may be greatly increased with success. The attempt to photo- 

 graph the corona during the partial phase is photographically 

 but a special case of the same problem. The difficulty was not, 

 indeed, to give a long enough exposure, but to avoid the fogging 

 of the plate from the brilliance of the returning sunlight and 

 from the consequent general illumination of the atmosphere. 

 The success of the attempt was most remarkable, and could only 

 have been obtained on a plate possessing the characteristics of 

 the triple-coated Sandell. When it is borne in mind that the 

 exposure given was equivalent to 9 seconds with //lo, or ,540 

 units ; that carefully backed plates exposed during mid-totality 

 for periods corresponding to this are strongly halated, and 

 that, surface for surface, the sun is something like 100,000 

 times as bright as the corona, it is indeed a remarkable 

 result to find that the coronal ring can be traced round 

 practically to meet the cusps of the returning sun, and that 

 the limb of the moon is seen sharply and distinctly defined 

 black against its bright background. With no other plate 

 that I know of would it have been possible to develop a 

 photograph with any hope of success upon which the image 

 of the sun had come out deep, distinct and black before ever 

 the developer touched it, Mr. Cousens, for example, exposed 

 a sixth plate, carefully backed, which was caught by the first 

 ray of returning sunlight. The flood of sunshine made it 

 impossible to fully develop the plate without entirely fogging 



