1868,] On Physical Observations to be undertaken in India. 289 



in the object viewed, both images become strongly coloured with comple- 

 mentary tints. 



"The second analyzer is formed of a Nicol's prism and a system of quartz 

 cut obliquely, known as Savart's polariscope. 



" It is almost certain that the light from the objects will be polarized 

 in a plane passing through the radius of the sun, if at all ; and arrange- 

 ments may be made accordingly. Nevertheless, if time permits, a rough 

 determination of the plane of polarization, in case the light should prove 

 to be more or less polarized, would be desirable. The plane may be deter- 

 mined roughly by the first analyzer by noticing the azimuth of the analyzer 

 when either image (specifying which) assumes the 'tint of passage' (a purple 

 in which blue and red are equally balanced), — more accurately by the 

 second, by observing the azimuth at which the bands disappear, and stating 

 whether on turning in a given direction from this position the bands seen 

 are black-centred or white-centred, or, which is better, when the po- 

 larization is but slight, by observing the azimuth at which the bands are 

 most vivid, and stating the character (black or white) of the central band. 



" § III. Instructions as to the general method of the observations to 



be made. 



"i. In order that the observer may acquire the necessary familiarity 

 with the use of the spectroscope, it is recommended that he apply himself 

 as far as his other engagements may permit, during the time after his arrival 

 in India until the period of the eclipse, to a prismatic examination of the 

 brightest of the Southern Nebulae. 



" The instruments sent out by the Royal Society are in every respect 

 suitable and convenient for these observations. The determination of the 

 character of the spectra of the more brilliant of the Southern Nebulae would 

 be a service for science of very great value. 



" It is recommended that the observer provide himself with a list of, say, 

 from fifty to a hundred of the brightest of the Nebulae the distance of 

 which from the north pole prevents an advantageous study of them in the 

 latitude of England. From this list the observer would select each night 

 the Nebulae which, at the time of observation, were situated near the meri- 

 dian. The equatorial mounting, with its finely divided circles, would make 

 the finding of the Nebulae a task of no difiiculty. 



" The observer should first make a diagram and general description of 

 the nebula as it appears in the telescope. The object should next be ex- 

 amined with the spectroscope added to the telescope. The telescope should 

 then be moved so as to bring in succession upon the slit the different parts 

 of the nebula. The wire arranged for that purpose in the finder would 

 enable the observer to determine with accuracy the part of the nebula 

 under examination. 



''At the commencement of the observation the slit should be widely 

 open, and the observer should then make it as narrow as the light of the 



