Total Solar Eclipse of 1900 (May 28). 355 



Hence to compare with a 1 second exposure to the corona through 

 the lens, we should expose the plate to the candle for 3~ seconds. Since 

 the faintest of the standard squares obstructs some of the light, and 

 since it is advisable to have an exposure on the plate from the standard 

 light rather denser than the densest part of the image, the plates 

 in slide No. 1 (to be exposed 1 second to the corona) were exposed for 

 6 seconds to the candle. 



Those in No. 2 (to be exposed 5 seconds to the corona) were exposed 

 for 40 seconds to the candle, and those in No. 3 (20 seconds to 

 corona) for 21 minutes. These exposures are even longer, relatively, 

 than No. 1 ; but on previous occasions the squares had not been 

 dense enough, and it was considered advisable to make sure of going 

 beyond the point required. 



21. Use of Green Screen. In slide No. 4 a coloured glass screen, 

 Ttindly provided by Mr. Shackleton, was placed in front of the plate, 

 with a view of obtaining information on the distribution of coronium 

 in the corona by comparing a photograph taken in green light with 

 the others. But owing to the following circumstance this particular 

 experiment was not a success. The plate should of course have been 

 one sensitive to green light, and some Cadett " spectrum " plates were 

 taken out to Algiers for the purpose. They were whole plates and 

 required cutting down to fit the slide. In the stress of other work the 

 experiment was forgotten until an hour before totality. There was 

 still plenty of time to fill the slide, and I went to the dark room to do 

 so. But the circumstances were scarcely favourable for manipulating 

 the diamond in the dark, as these plates require. The first plate broke 

 and cut my finger, not seriously, but enough to hamper me, so that I 

 had no better success in cutting another plate. Indeed, after one or two 

 attempts I had to give it up. It seemed just worth while putting 

 in a " Rocket " plate behind the green screen, but there was very little 

 on the plate when developed ; and the experiment was on this occasion 

 of little or no value. I am sorry to have been unable to do justice to 

 Mr. Shackleton's kindness in providing the screens, and hope that on 

 another occasion I may make better use of them. 



22. An Integral Photometer. To obtain an estimate of the total light 

 given by the corona, an Ilford " Empress " plate was exposed to its 

 light for 10 seconds, in a small camera from which the lens had been 

 removed, so that the corona shone directly on the plate, but side light 

 was excluded. On the same plate standard squares were impressed by 

 exposing it to the candle at 5 feet, as in 8. The exposure given to 

 the candle was, by an oversight, not recorded, but was either 10 or 

 20 seconds. The oversight was discovered before the eclipse, and 

 another plate from the same batch was exposed to the candle and 

 squares for 5, 10, and 20 seconds, and developed in the same dish. The 

 effect of the corona was, however, considerably greater than that 



