292 On Physical Observations to he undertaken in India. [Mar. 5, 



" A small observing-hut, or temporary place of shelter, would be pro- 

 bably necessary. 



" It is recommended that the eyepiece magnifying twenty-seven dia- 

 meters be used. 



" The observer should fix upon the eye-end of the telescope a disk of 

 cardboard some 12 or 15 inches diameter, which, near the edge, may be 

 roughly divided by a few large figures, which can be easily read in the 

 feeble light which prevails during the totality. To the small tubes carry- 

 ing the analyzing prisms a long index of card should be attached. In 

 this way the plane of polarization may be read off notwithstanding the 

 feebleness of the light. 



With a dark glass the observer is to watch the progress of the eclipse 

 until the whole of the sun is obscured. The dark glass is then to be re- 

 moved, and the corona and red flames observed for traces of polarized light. 



" There are two analyzers provided. 



" The observer is to use first the double-image prism and plate of quartz. 

 A slight degree of polarization will show itself by a difference of colour in 

 the two images. 



" An attempt should then be made to determine approximately the 

 plane in which the hght is polarized. 



If polarization is detected in the 'corona,' or in any prominence of 

 large extent, the second analyzer may probably be employed with advan- 

 tage. This consists of a Nicol's prism and a compound plate of quartz, 

 showing Savart's bands. By means of these bands, the plane of polariza- 

 tion of the light analyzed may be easily obtained." 



The observation of the spectra of the corona and red protuberances 

 being one of considerable delicacy, it was highly desirable that the ob- 

 server should have some previous training in this country, while, on the 

 other hand, to send such an observer on purpose would involve consider- 

 able expense. It fortunately happened that Lieut. J. Herschel was at the 

 time in this country, and was about to return to India in November 1867, 

 to resume his duties in connexion with the great Trigonometrical Survey 

 of that country, Mr. Herschel took a lively interest in the subject, and at 

 once, on being applied to, undertook the observations which were to be 

 made with the large telescope furnished with the spectroscope, subject to 

 the approval of Colonel Walker, Director of the Survey. This approval 

 was readily given, as will appear from the following letter : — 



" Colonel Walker to General Saline, August \ 7th. 



" Debra Doon, via Bombay, 

 17th August, 1867. 



My dear Sir, — I have to acknowledge your letter of the 30th June, 

 forwarding a copy of the Report of the Committee appointed by the 

 President and Council of the Royal Society, to take into consideration 

 Mr. Hennessey's letter of the 13th February, 1866. 



