Page:Proceedings of the Royal Society of London Vol 69.djvu/249

Rh surface 3| x 21 inches, was fixed on a turn-table in front of a tele- scope of aperture 3| inches, and focal length 29 inches. A positive eye- piece with two cross- wires was used, which gave a magnifying power of 19 '2, and whose circular field of view was rather more than 1 in diameter.

The instrument was mounted so that the telescope was parallel to the earth's axis and pointed to the south pole. The grating was used in a manner analogous to that in which the mirror of a polar heliostat is used. The light of the corona was incident at an angle of about 57, and the diffracted beam utilised in the telescope left the grating at an angle of about 13. In this position of the grating, the green of the second order was used and the magnifying power of the grating- was approximately one-half, so that the distorted coronal ring was an ellipse, of which the major axis was about twice as long as the minor axis, the minor axis being parallel to the length of the spectrum and perpendicular to the direction of daily motion. No clockwork was used, but a wedge pushed under a wooden arm fixed to the instru- ment and projecting radially outwards from ithe telescope served to give a satisfactory slow motion by slightly turning the whole instru- ment about what may be described as polar pivots.

The instrument was focussed on the E group of lines about two minutes before totality began. The crescent was small enough to show the lines sharply, though the instrument was so set that the point of second contact was near the end of the major axis of the 1 distorted image.

When I came to make observations of the green coronal ring, I first looked very carefully for fine radial structure ; and finding none, 1 altered the focus very slightly. This did not improve matters, so 1 reverted to the original focussing, and am quite satisfied about the observation, which could not be more definite.

The observations were begun after the completion of the third minute of totality and were completed in about 80 seconds. Two ellipses had been drawn beforehand to serve as outline for the distorted limb of the moon ; and on one of these the record of contour lines of the brightness was quickly but carefully sketched.

Results. The coronal ring was distinctly seen, but there was a good deal of other green light throughout the whole of the field of view, doubtless from clouds. No fine radial structure could be seen any- where round the ring ; but there were two broad patches of the light of " coronium " projecting from the elliptical ring.

The accompanying figure (fig. 2) shows by the dotted lines the ellipse drawn beforehand, and the contour lines as sketched in during the eclipse. The unbroken lines indicate the results got by projecting the ellipse into a circle and making the corresponding change in the contour lines.