MOUNT WILSON SOLAR OBSERVATORY. 191 



shown on the larger scale photographs, and the structure of individual lines 

 is seen much more clearly. Accordingly, the classification is based for the 

 most part upon a comparison of groups of lines as close to each other as 

 possible, and nearly of the same intensity and structure. In the earlier B-type 

 stars, for example, the so-called "Orion" lines are compared with the hydro- 

 gen and the helium lines, while in the later B stars the relative strength of 

 the helium line A 4472 and the magnesium line A 4481 furnishes an extremely 

 important and valuable criterion. The two lines are equal in stars of type 

 B 8. In the A-type stars the line A 4481 is compared with neighboring metal- 

 lic lines as they begin to appear. A gradual increase in the number and in- 

 tensity of these metallic lines is the distinguishing feature of the transition 

 from A-type stars to F stars. With the F-type stars the hydrogen lines 

 begin to decrease in width and intensity and can be compared with neighbor- 

 ing metallic lines. Care has been taken in such comparisons to avoid en- 

 hanced lines and other lines of a peculiar character. 



The number of stars so far classified is 453, and the results are based upon 

 about 1,500 spectrograms. Seventy of this number are stars with known 

 parallaxes and mainly of types F to M. The remainder are for the most 

 part of types B and A and are distributed in the following subdivisions : 



The high dispersion of the spectrograms made it possible to detect numer- 

 ous peculiarities or differences in the spectra of stars of the same type. Es- 

 pecially important is the distinction between stars having sharp and those 

 having hazy lines. A large proportion of the latter have proved to be spec- 

 troscopic binaries and this fact may be accountable for the character of the 

 spectrum. Some prominent groups of peculiar spectra are as follows : 



(1) Thirty-two stars which show very strong enhanced lines, sharp, for 

 the most part. A provisional examination of the stars with known paral- 

 laxes seems to indicate that the stars with strong enhanced lines have a 

 greater absolute luminosity than those not showing the enhanced lines 

 strongly. 



(2) Fourteen stars with very sharp lines. These belong to group c of 

 Miss Maury's classification. 



(3) Fifteen stars which show bright hydrogen lines. 



(4) Eleven stars which show composite spectra. All of these stars are 

 spectroscopic binaries, and in all cases the brighter component shows the 

 earlier type spectrum. 



The results of the classification agree closely with the Harvard results. 

 There appears to be a slight systematic difference in the case of the earlier 

 B-type stars, the stars with sharp lines frequently being put one or two sub- 



13— YB 



