Page:Proceedings of the Royal Society of London Vol 69.djvu/245

Rh eclipsed sun could be seen through them. Each was mounted in a turning tube, provided with a pointer and graduated circle and attached to a wooden stand.

The programme of observations was that I should look through A as soon after the beginning of totality as the adjustment and exposures of the 4-prism spectroscope would set me free. The polariscope was to be then turned in its tube until the Savart bands were invisible, and

I was at once to set the large photographic Savart camera to the corresponding reading, and to call out to Mr. Wallace, who was making the exposures for that camera.

Next the polariscope B was to be used half-way through totality, when my visual observations with the grating-spectroscope were com- pleted. Finally the polariscope C was to be used towards the end of totality, just before the fifth minute was completed. Each of the polariscopes was to be left as adjusted " for extinction," and the readings were to be made at leisure after the eclipse, except in the case of A, of which the readings were needed at once.