290 On Physical Observations to be undertaken in India, [Mar. 5^ 



object would permit. In this way the character of the spectrum of each 

 portion of the nebula could be recorded in connexion with the description 

 of its appearance which had been already made. If the spectrum should 

 be discontinuous, the positions of the bright lines should be measured by 

 the micrometer-screw, or by the reflected scale. It is recommended that 

 the screw, should be used in preference to the scale whenever it is found 

 practicable to do so. 



For the purpose of obtaining the value of the micrometer-screw and 

 of the scale, measures should be frequently taken of the principal lines of 

 Fraunhofer. These measures should be taken as near the time when the 

 observations are to be made as possible. As a precaution against any 

 accidental displacement of any part of the instrument, at the time of ob- 

 serving the nebulae, a reading should be taken of the sodium line (D of 

 Fraunhofer) by means of a small alcohol lamp placed before the object- 

 glass of the telescope. 



" A series of observations of the principal Southern Nebulae would be 

 of very great value in the present state of our knowledge, and would cer- 

 tainly repay the entire cost of the instrument, should bad weather, or some 

 unforeseen accident render the primary object, the investigation of the sun, 

 impossible. 



" The observer should also practise himself with- observing the spectrum 

 of the moon, and of a cloud brightly illuminated by the moon, in order to 

 become familiar with the appearance of spectra which are continuous, 

 except so far as they may be interrupted by dark lines, and which come 

 from objects rather deficient in illumination. 



. Instructions for the spectrum-observations of the eclipse. 



" The equatorial telescope should be previously put up accurately in 

 position, within a suitable temporary hut or observatory. The clockwork 

 is to be adjusted to sun's apparent motion. The Barlow lens is to be 

 employed, and placed within the focus at a distance sufficient to double, 

 or nearly so, the diameter of the sun's image. 



" Great care must be taken that the finder is in perfect adjustment, so 

 that the spectrum of any object brought upon the point of the indicating 

 wire may with certainty be visible when the eye is placed to the little 

 telescope of the spectrum-apparatus. 



" It will be well for the observer to decide previously whether he intends 

 to make use of the micrometer- screw, or of the illuminated scale. Still he 

 is requested to have the small lamp lighted, and both methods of measuring 

 in proper order, so that either could be available instantly during the 

 duration of the totality. 



" During the progress of the eclipse, both before and after the totahty, 

 the observer is to take measures of the lines of Fraunhofer, with the micro- 

 meter, and also with the scale. 



" The measures obtained of the eclipse, and which have been made in 

 the interval between the two sets of observations of the solar lines, can 



