Page:Proceedings of the Royal Society of London Vol 69.djvu/273

Rh graphs were taken of the Sun, on the same plate for orientation. The plate was an Imperial Fine Grain Ordinary and the exposure was as quick as could possibly be given by uncovering and covering the object- glass by hand. The true local times of the three exposures were :

h. m. s.

7 58 39 3

7 58 57-3

7 59 17-3

A ninth plate, a Wratten and Wainwright Instantaneous plate, was exposed with the same aperture and exposure at 8 h O m 17 S> 3. The plate in the tenth plate-holder was not used.

Two sets of " Abney squares " were printed upon No. 4 on June 13, before the plate was developed. Both sets were printed by the light of a Sugg's Standard candle, at 5 feet distance ; the one being exposed for 15 seconds, and the other for 4 minutes.

No. 1 is a clean and fairly dense negative, showing well the polar rays and the structure of the lower corona, especially in the east equatorial wing. The west wing shows less detail. The corona is traceable to a distance of 15 or 16 minutes from the limb of the Moon in the S.E. ray.

No. 2 is a clear thin negative showing much about the same detail as No. 1.

No. 3 is a thin negative but shows a considerable extension of the great S.E. and N.E. rays and a good deal of detail in their lower regions.

No. 4 is a dense negative showing the greatest degree of extension in the coronal streamers of any of the series. In the case of the two chief rays, the corona can be traced to a distance of nearly half a degree from the Moon's limb.

No. 5 shows the lower corona well up to 5 or 6 minutes from the Moon's limb.

No. 6 is partly spoiled by fog, and only the lower corona is seen up to about 3 minutes.

No. 7 is a faint and delicate negative showing the chromosphere, prominences, and the lower corona up to a distance of about 3 minutes from the limb of the Moon.

Two methods were employed to focus the Greenwich coronagraph ; the first being the method described by the Astronomer Royal in his Reports of the Eclipse Expeditions of 1896, 1898, and 1900,* an image of an object (gauze net in the plane of the plate) being photo- graphed by reflection normally from the plane mirror of the crelostat. The second method was by photographing Arcturus, the image of which was allowed to trail across the platel

and Tol. 60, p. 397.
 * 'Roy. Soc. Proc.,' TO!. 64, p. 8 ; and ' Monthly Notices, E.A.S.,' TO!. 57, p. 105,