Page:Proceedings of the Royal Society of London Vol 69.djvu/239

Rh shutter sticking rather. Then re-set bright side of corona according to picture [previously made to indicate position angle of crescent on slit], and began to expose the five remaining plates in 4-prism [for second ' flash ']. At 5 m 35 s I called ' Now ' to Briggs and Mrs. N. for their final photographs, when I felt end of totality had come ... [I turned to chronometer and waited for] W.'s signal ' Now,' which was to mean the 15-seconds crescent [of returning sun]. I noted this on chrono- meter, and waited further for Hobart's final ' Sixty,' which I also noted on the chronometer. Called ' Cover slides,' and then went out of hut and saw mackerel cloud all over the sky near the sun and 4 glory ' rings coloured round crescent. Sky got rapidly more clouded, and clouds drifted slowly towards west. Collected notes from observers."

Summary of Exposures. Six exposures were made near second con- tact with the following durations :

1 second, 1 second, 2 seconds, 2 seconds, 2 seconds, 3 seconds.

The seventh plate was exposed for the corona spectrum for 2 m 18 s. Five exposures were made near third contact with the following durations :

2 seconds, 2 seconds, 1 second, 1 second, 1 second.

Results. Of the twelve plates thus exposed, only four plates show any impress of the light. The rest, including the exposure on the corona, show nothing, though development was pressed as far as possible.

Five minutes before totality a photographic plate was exposed for 15 seconds to get the spectrum of the sunlight reflected from the clouds near the crescent this was to serve to give standard measure- ments to which the measures of the corona spectrum were to be referred and a good strong negative was obtained.

The plates at second contact are very feeble negatives, and I attribute their weakness to 4 cloud. Photographs obtained with the other instruments at the beginning of totality show that at least three- quarters of the light was intercepted by cloud.

The spectrum of the corona failed because of insufficient exposure under the cloudy conditions.

The plates at the end of totality, when there was certainly very much less cloud, show no sign of impress of the light, and I can only explain it by assuming that the image of the reappearing crescent must have been set across the slit in such a way that three or four more exposures than I had the means of giving were needed to catch the reappearing chromosphere. Photographs obtained with the objective grating show that there was plenty of light to impress the plate, if only it had passed into the slit.