1868.] On Physical Observations to he undertaken in India, 291 



thus be referred to them, and the positions on the spectrum of the lines 

 accurately determined relatively to the lines of Fraunhofer. 



*' The finder is provided with, a diagonal eyepiece, over which a wedge 

 of dark glass is made to slide, 



" The observer is expected to watch the progress of the eclipse in the 

 finder, moving the wedge of glass as the light of the sun diminishes. 



" As soon as he perceives a * red prominence,' by means of the very 

 efiicient arrangements provided in the instrument, he is to bring the red 

 prominence upon the point of the indicating wire. The observer will then 

 see the spectrum of the * red flame ' in the little telescope of the spec- 

 troscope. 



At the commencement of the observations the lowest eyepiece should 

 be used, and the slit should not be too narrow. 



The observer is first to record the general character of the spectrum, 

 whether continuous or discontinuous. 



" Then the principal lines, whether dark or bright, are to be measured 

 with as much care as the very limited time will permit. 



" As the spectrum of the red prominence is compound, and contains the 

 spectrum of the light of the portion of corona before it (possibly also to 

 some extent of the corona behind it), sufficient time must be left to move 

 the instrument so as to bring upon the slit a part of the corona where it is 

 brightest. 



" The character of the spectrum must be in a similar manner examined, 

 and any lines present measured. 



" Of course, if there should be found time to do so, it would be de- 

 sirable that several red prominences should be examined, and also light 

 from different parts of the corona. The observer, however, is strongly 

 recommended to make as complete an analysis as possible of the *red 

 flame' first selected. 



" The observer is requested to send in full all the details of the ob- 

 servations as they were taken down at the time. 



" A Clerk will be required to write down the results at the moment from 

 the dictation of the observer. 



*' iii. The use of the portable hand spectrum-telescopes will be obvious. 

 The observer has only to direct the instrument to the sun at the moment 

 of totality. The instrument should be previously focused to suit the ob- 

 server, upon the moon or some distant object. The light of the corona 

 and red flames will be dispersed into its component colours. It will be 

 easily detected whether the spectra of the corona and of the red flames are 

 continuous, or consist of bright lines. The four instruments should be 

 placed in the hands of observers stationed at different places along, or 

 nearly along, the central line of the eclipse. 



iv. Observations for polarized light in the corona and 'red flames.' 



" A distinct observer is required for these observations. He should be 

 famihar with the telescope and its motions. 



