162 THE INDIAN ECLIPSE, 1898. 



second. Mr. Thwaites' exposures were 1J, 9, and 30 seconds, 

 corresponding approximately to 1*2, 7*25 and 24*2 seconds with 

 //1 5. Mr. Bacon's exposures were 1 second and 2 seconds, his 

 f/a being almost 15. But both observers used Ilford ordinary 

 plates, not extra-rapid. The exposures are therefore equivalent 

 only to one-fourth or one-fifth of what an ultra-sensitive plate 

 w r ould have given. Mr. Tlrwaites' second plate is very fully 

 exposed, one of the coronal streamers at least reaching the edge 

 of the plate ; and though it shows a great wealth of detail, yet 

 over-exposure is beginning to show itself near the limb. Mr. 

 Bacon's two plates are both amply exposed, whilst Mr. Thwaites' 

 third plate is evidently not at all the equal of his second. 



It may be remarked that the two observers were well advised 

 to use slow plates ; for where there is no urgent need to cut the 

 exposures very short, there can be no doubt that it is the wiser 

 course to use a plate of normal sensitiveness, rather than an 

 extra-rapid, and to lengthen the exposure in proportion. The 

 slower, plate is easier to manage and safer to handle ; in other 

 words, a restrained and prolonged development can be better 

 employed upon it, and it is less liable to accidental fog ; whilst 

 the grain of the deposit is usually finer. Certainly the five 

 plates in question leave nothing to be desired as to detail and 

 beauty. 



The fourth and fifth of the Cousens series show a very real 

 development of the coronal rays : so far as we are aware, the 

 greatest development shown on any photographs obtained during 

 the eclipse, except the three which we secured with the Dallmeyer. 

 These two Cousens plates had exposures of 2 and 4 seconds 

 respectively, equivalent to 120 and 240 units. The first of the 

 Dallmeyer series would have been 360 units, but it is spoiled 

 by bad shake. The second and third had exposures 1800 and 

 7200 on the same scale, or 31 seconds and 125 seconds with 

 //1 5. The advantage in extension of the second Dallmeyer over 

 the fifth Cousens is small, when it is borne in mind that the 

 equivalent exposure is 7J times as long. The two plates repre- 

 senting the third stage in the Dallmeyer series carry the rays 

 very considerably further, but their exposure was 30 times that 

 of the fifth Cousens, and they were taken on the Sandell triple- 

 coated plates. 



It is precisely in these extreme exposures that the value of 

 the Sandell plate becomes apparent. The problem of photo- 

 graphing the coronal extensions has three elements. The first, 

 the extreme faintness of these rays, can, of course, be overcome 

 by increasing the equivalent exposure in each or all of three 

 different ways, that is by increasing the actual duration of the 

 exposure, by increasing the ratio of aperture to focal length, 

 or by increasing the sensitiveness of the plate. The second 

 element is the presence in close contiguity to these faint rays 



