STOKES. — SPECTROSCOPIC OBSERVATIONS. 25 



the Sun remains at a low altitude every day shortly after he makes 

 his first appearance in the spring affords time for more deliberate- 

 observations than can be made during the few minutes he remains 

 at a low altitude in places in comparatively low latitudes. 



The Expedition will be furnished with a spectroscope of high 

 dispersion, and with maps of the spectrum showing the additional 

 bands seen when the Sun is low. 



The best time for observation will be in the spring, shortly 

 after the first appearance of the Sun, or, if circumstances should 

 allow, in the autumn, shortly before he disappears for the winter, 

 inasmuch as in either case the Sun will remain for a comparatively 

 long time at a very low altitude. 



As the appearance of the spectrum changes a good deal with 

 the degree of detail in which it is seen, the spectrum should, in a 

 preliminary trial, be compared with the maps, and that map more 

 especially worked with which best matches the object. The 

 power of the spectroscope might even be reduced by the removal 

 of one or two of the prisms, if a better match as to degree of 

 detail can thus be obtained. 



For the actual observations, days should be chosen when the 

 Sun is clear down to the horizon. The object is to be com- 

 pared with the map or maps selected, going regularly through a 

 portion of the spectrum each day as time permits, and making 

 memoranda of the accordance or otherwise of the object with the 

 map. Measurements need not in general be taken, except when 

 the identification of a line is doubtful. Should the relative 

 strength of any terrestrial line as compared with its fellows appear 

 distinctly different in the object from what it is in the map, such 

 line should be marked for re-examination. 



The lines so marked should subsequently be re-examined at 

 various low altitudes of the Sun. It is to be remembered that 

 the various lines or bands of absorption as seen under otherwise 

 given conditions and at a given place, do not all increase in the 

 same proportion as the Sun approaches the horizon ; so that the 

 apparent abnormal strength of any particular line as compared 

 with its fellows which had been noted in the first instance might 

 be due to its having been seen at a difierent altitude of the Sun 

 from that to which the map relates. 



The probability of the discrepancy between the object and 

 the map being thus explicable is to be judged of by the result of 

 the re-examination. Should it appear at all probable that the 

 result is not thus to be explained, the line should be noted by 

 reference to the map, or if its identification should be at all 

 doubtful, its distances from two neighbouring easily identifiable 

 lines right and left of it should be measured, and its breadth 

 measured or estimated with reference to some other line, the 

 appearance of which agrees with its representation in the map. 



Should any of the lines, even after the re- examination remain 

 apparently discrepant from the map, it would be well, if leisure 

 permit, to examine them again further on in the season under 

 difierent conditions of temperature, direction of wind, &c., and 

 note whether any change is observed. 



36122. c 



