166 THE INDIAN ECLIPSE, 1898. 



during the partial phase. There can be no doubt that plates 

 triply-coated, or possibly even with four coats, will be necessary 

 for this work. It seems to us that the chief improvement 

 possible upon our attempt during the late, eclipse lies in the 

 direction of very greatly shortening the exposure. There can 

 be no question at present of photographing the long rays or 

 the outer corona in a large amount of sunshine. The only 

 thing that can be hoped for at present is to secure the inner 

 brighter corona. Now for this under favourable conditions 

 _i_ or i second with //1 5 is sufficient during totality ; out of 

 totality a yet shorter exposure will be enough, and it will be 

 well to arrange for a fairly wide range of exposures say from 

 i second down to -g 1 ^. In all probability it will be found that 

 the duration of the most efficient exposure varies with the 

 amount of the sun's disc uncovered, and that the farther out of 

 totality the photograph is taken, the shorter should be the 

 exposure. 



The question of hiding the sun behind an occulting disk 

 may offer some little difficulty ; but if a sufficiently good 

 guiding telescope is rigidly attached to the camera, and the 

 disk is the precise size of the image of the sun, it will obviously 

 be a great advantage to use one ; but all would depend as to 

 whether the sun could be brought accurately upon the disk at 

 the moment of exposure. This is a crucial point. 



The importance of using a disk does not lie only in the fact 

 that it would screen the plate from the direct rays of the sun. 

 It will be seen, on referring again to the little photograph on 

 p. 125, that not only was the sun's image reversed, but it was 

 solarised ; and though we m#y hope to diminish the cause of 

 this on the one hand by immensely shortening the exposure, 

 yet, on the other hand, if we succeed in getting photographs 

 far out of totality, the larger amount of the sun's surface visible 

 would increase it on the other. At all events the presence of 

 this solarised ring is fatal to any chance of showing the corona 

 on the sunlit side of the partial eclipse. On the other hand, 

 the moon itself would cover the whole of the inner corona on 

 the other side except at the cusps and when the partial phase 

 was not far short of totality. This difficulty is not insur- 

 mountable, as photographs of the entire disk of the sun have 

 been taken with an ample exposure for securing the corona, and 

 yet without any trace of solarisation at the limb ; but it requires 

 very skilful manipulation to avoid it, and the use of an occulting 

 disk would seem to offer the easiest way of securing the desired 

 result. 



A. S. D. MAUNDER. 

 E. WALTER MAUNDER. 



