158 THE INDIAN ECLIPSE, 1898. 



ON THE PROPER EXPOSURE TO BE GIVEN FOR PHOTOGRAPHS 

 OF THE CORONA.* 



ONE of the most interesting features in the observations of 

 the late eclipse is the enormous variation in the length 

 of the exposures given. To take the two extreme cases, Prof. 

 C. Michie Smith, with an objective of 6 inches aperture and 

 40 feet focal length, that is with //80, exposed one of his plates 

 for half a second. Mrs. Walter Maunder, with a lens of H inch 

 aperture and 9 inches focus, i.e. //6, exposed two plates for 20 

 seconds each, If we take it that the equivalent exposure varies 

 inversely as ({) 2 , then in the second instance the exposures were 

 more than 7000 times as great as in the first. 



Since both these extreme exposures were successful for the 

 respective purposes for which they were designed, it would 

 seem, at first sight, as if any exposure between these two 

 extreme limits might serve. And there can be no doubt that 

 a really skilful photographer, having a clear idea of the practical 

 exposure which he had given, and of the special features which 

 such an exposure was well calculated to bring out, might even 

 succeed in producing a negative not without value with any 

 exposure within this wide range, possibly within a range even 

 wider still; provided always, of course, that the sky was really 

 clear, and that there was an absence of anything like over- 

 whelming atmospheric glare. 



In the Indian eclipse of 1898, the weather conditions were 

 everywhere so favourable that this condition was fulfilled 

 throughout. The question, therefore, was one of pure photo- 

 graphy, and we may ask whether we have any indications as to 

 what are the suitable exposures to be given in order to secure 

 certain definite pictures of the eclipse. 



Accepting the principle that with different instruments the 

 duration of exposure should vary inversely as (f/a) 2 , in order 

 to produce the same result, we have before us three series of 

 negatives that may give some definite information. The first 

 we will call the Waters series, as it was taken with the tele- 

 photographic camera bequeathed by the late Mr. Sidney Waters 

 to the Royal Astronomical Society. Here the equivalent f/a 

 was 57, and the exposures given were 1, 5, and 20 seconds. 

 The second series was intended as a continuation of this, and 

 was taken by Mrs. Walter Maunder with the Dallmeyer stigmatic 

 lens of //6, the exposures being the same as in the Waters 

 series. This we will call the Dallmeyer series. The third is a 

 set of five plates taken by Mr. Henry Cousens, which we will 

 call the Cousens series. The f/a in this series was 15, and 

 the exposures were J, J, 1, 2, and 4 seconds. The value of a 

 given exposure in the Waters series was double that of the 



* By E. Walter Maunder, F.R.A.S. 



