220 Mr. H. F. Newall. 



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 



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

 58, App.,p. [55]. 



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

 App., p. [20]. 



