Page:Proceedings of the Royal Society of London Vol 69.djvu/241

Rh In the event, the long exposure was sacrificed, the other five were made.

The actual exposures were :

1. 8 seconds, beginning 10 seconds after the signal "Go."

2. 17 ,, 2 minutes 35 seconds after the signal " Go."

3. 36 3 15 (? 5 seconds).

4. Omitted.

5. 4 seconds, beginning 5 minutes 3 seconds after the signal " Go."

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The first exposure was made through clouds.

The interval between the first and second exposures was more than 2 minutes longer than it would have been had the sky been clear. In the third exposure one of the shutters had evidently not acted, the photograph showing nothing but the standard squares, which had been put on just before development. The last two exposures were carried out according to programme, and were made probably at moments when the sky was clearer than at any other time during the whole eclipse.

The Standard Squares. After the eclipse, Sir. W. Abney's standard squares were impressed on all the plates for photometric measurements. Exposures were made to a standard candle at 5 feet from the plate, and, following a suggestion made by Professor Turner after his experience with the plates taken at Algiers in 1900, two sets of squares were impressed, with different exposures, on all the plates, as follows :

Exposure for Exposure for standard

Plate. corona. squares.

No. 1 8 seconds 10 seconds and 5 seconds

2 17 20 5

3 36 80 5

,, 4 not exposed 80 ,,

,,5 4 seconds 80 ,,

6 2 20 5

Development. All the plates were developed with metol.

When it was found that there was no image on No. 3 one of the exposing shutters evidently had not acted it was thought wise to impress squares on No. 4, and develop it, just for the very improbable chance that it had been exposed in mistake for No. 3, but no image appeared on the plate.

Results. The four photographs obtained are all good clean plates. The effect of the cloud is clearly seen in the plates, though it is surprising that the definition has not been more affected by it. The first exposure (8 seconds) shows less than the last (2 seconds) ;