120 



Lieut. J. HerschePs Account of the 



[Nov. 19, 



44. Mr. Chambers prevented by Clouds from using two other 

 Spectroscopes. 



Two other hand spectroscopes in my possession were lent to Mr. 

 Chambers, Government Astronomer at Bombay, who stationed himself not 

 far from Begapur ; but I am sorry to say he was denied the opportunity of 

 using them by the clouds. 



Gentlemen, I beg to apologize for the length of my narrative, and to 

 subscribe myself, with much respect, 



Your obedient Servant, 



J. Herschel, Lieut. R.E. 



Bangalore, Sept. 1868. 



LIEUT. CAMPBELL'S REPORT. 

 " I was deputed to accompany Lieut. Herschel on his expedition to ob- 

 serve the phenomena of the total eclipse, and to use the instruments sup- 

 plied by the Royal Society for the observation of polarized light in the 

 corona and red flames. 



"The instruments in question were as follows: — A telescope of 3-inch 

 aperture, mounted on a rough double axis, admitting of motion in azimuth 

 and altitude by hand only, unaided by any appliance for clamping and 

 slow motion. The telescope was provided with three eyepieces of magni- 

 fying-powers 27, 41, and 98 ; and with it were furnished two analyzers for 

 polarized light, viz. a double-image prism and a 'Savart's polariscope.' 



" The first gives two images of the object viewed, which, when polarized 

 light is present, become strongly coloured with complementary tints, by 

 whose changes, according to the position in azimuth of the analyzer, the- 

 plane of polarization may be found. 



" The second shows the presence of polarized light by the formation* 

 across the image of the object viewed, of coloured bands, which alter in 

 arrangement and intensity according to the position of the polariscope with 

 reference to the plane of polarization, and hence afford a means of arriving 

 at a knowledge of the latter. 



"With the former, slight polarization would probably be more readily 

 recognized at a glance ; while with the latter the plane of polarization could 

 be more easily and accurately determined. 



"To carry these I had a pair of jointed arms constructed, so at- 

 tached by a collar and screw to the eye-tube of the telescope as to admit 

 of the eyepiece being changed. Each arm carried one of the analyzers in 

 a cell, in which a rotatory motion could be given for analyzing purposes. 



" Either analyzer could in this way be brought instantly into position 

 before the eyepiece of the telescope, or both could be turned aside and the 

 telescope used by itself at pleasure. 



" Immediately behind the apparatus a circular piece of cardboard of 

 about 12 inches diameter and neatly graduated was firmly attached to the 

 eye-tube, and to each analyzer was affixed a long pointer by which its- 



