390 Mr. J. N. Lockyer. [Feb. 28, 



is very nearly as strong as B. The appearance presented to the eye 

 by the real trio in stars of Group III is accordingly very different 

 from that presented by the three lines in stars of Group V. 



Besides the least refrangible member of the trio there are other 

 lines which are special to Group III. One of these lies between F 

 and 6, at wave-length 499, as nearly as can be determined with small 

 dispersion. In some of the stars this line is very strong. It is only 

 seen as a very faint line in Capella, Arcturus, or the Sun, and is con- 

 sequently an important criterion. The nearest line of anything like 

 equal importance in Group V stars is the iron line at 495' 7. 



Two lines, at 579 and 568 respectively, also appear to be special to 

 Group III. No lines of similar intensities are seen in either Capella, 

 Arcturus, or the Sun in those positions, although fainter lines are 

 seen. 



In Rowland's photographic map of the solar spectrum there is a 

 line at 5659 which is much stronger than the one nearest to 568, and 

 this is not seen at all in Group III stars. Only a very faint line is 

 indicated in the same map at 5791, there being a stronger line at 

 5763 which is not seen in Group III stars. The two lines at 568 and 

 579 are, therefore, special to Group III. The line at 579 was com- 

 pared directly with the low temperature iron line at 579, and the 

 coincidence established with the dispersion employed; this may, 

 therefore, be taken as due to iron. It may also be suggested that the 

 line at 568 is the double green line of sodium, which appears bright 

 in some of the bodies of Group I. Other lines referred to in the 

 observations are near 546 and 558, but it is not easy to distinguish 

 these from lines seen in stars of Group V. There are several strong 

 lines seen in the solar spectrum in the neighbourhood of 546, and 

 there are also strong lines at 5573 and 5587. In order to determine 

 whether these lines will serve as criteria or not, further inquiry with 

 greater dispersion will be necessary. 



The magnesium line 5527 appears to be common to both Groups 

 III and V, just as b is common to both. 



There seems to be no doubt, therefore, that criteria between Groups 

 III and V have been determined by the observations, and we are now 

 in a position to assign the stars of Vogel's Class Ha to one group or 

 the other according as the lines which have been shown to be special 

 to Group III are present or absent. 



One of the chief objects I have had in view in writing this paper is 

 to enable others to take up this important piece of work as soon as 

 possible when once the idea of increasing and decreasing temperatures 

 is generally accepted. 



