Page:Proceedings of the Royal Society of London Vol 69.djvu/236

220 mirror, made by Dr. Common and silvered by the Improved Electric Glow-Lamp Company, had a very fine surface, which was found in admirable condition when it was unpacked and put into its mounting on May 14. All the preliminary adjustments of the polar axis of the instrument had been made by means of the attached declination- theodolite, as in recent eclipses. Dr. Common kindly lent me a 6-inch flat, silver-on-glass mirror (also silvered by the above-named Company), and I used it once or twice before May 14, temporarily fitted to the mounting of the coelostat ; for it seemed safer in the tropical climate to run as little risk as possible with the 16-inch mirror, though I had, thanks to the kindness of Mr. Hinks, taken out silvering solutions in case of need. The final adjustment of the polar axis of the instrument was completed after the large mirror had been put in position in the mounting, and calls for no special mention.

The tilt of the mirror i.e., the perpendicularity of its normal to the polar axis was tested by making the polar axis horizontal, as judged by the readings of the level on the theodolite, and then com- paring the readings of a small level carefully placed on a sheet of paper on the silvered surface of the mirror, with the readings of the theodolite level. This procedure was regarded as sufficient and satis- factory for a place close to the equator, though exception might be taken to it were it adopted in higher latitudes.

11. The k-prism Spectroscope with two Slits.

It was intended to attempt

(i.) To secure five photographs of the bright-line spectrum of the sun's limb at the beginning of totality, and six others at the end, one slit only being used.

(ii.) To photograph the spectrum of the corona in both eastern and western equatorial regions to get material for determination of the relative motion in the line of sight (rotation of the corona), two slits being used.

The instrument arranged for this work is a 4-prism spectroscope with two slits. It was used by the writer in India at Pulgaon in 1898,* and at Algiers in 1900.1

The train of prisms is of such dimensions and construction as to transmit a 2-inch beam of light and to produce a minimum deviation of 180 for H y. The camera and collimator are set parallel to one another.

The whole spectroscope is mounted so as to turn about an axis

58, App.,p. [55].
 * 'Roy. Soc. Proe.,' vol. 64, p. 55; 'Monthly Notices, Boy. Astro. Soc.,' vol.

t ' Roy. Soc. Proc.,' vol, 67, p. 356 ; ' Monthly Notices, Roy. Astro. Soc.,' vol. 60, App., p. [20].