130 MR. F. McCLEAN ON THE COMPARATIVE PHOTOGRAPHIC 



/3 Lyree is also a helium star. Its spectrum belongs to Subdivision I. (b). Both the 

 hydrogen and the helium lines appear bright. The peculiarities of this star are well 

 known. 



A third bright line star, of another division, may be mentioned here. The spectrum 

 of Mira Ceti belongs to Type III. or Division V. The bright hydrogen lines appear 

 periodically, for a brief time, with great brilliancy. The banded absorption spectrum 

 brightens up simultaneously, a peculiarity which it is difficult to explain. The 

 exceptionally good photograph of this spectrum was taken on the 1st January last. 



Division II. is especially the hydrogen type. In it the hydrogen spectrum attains 

 its full development both in the strength of the lines and in the extent of the 

 spectrum. This is shown in the ultra violet series of lines, discovered by Dr. HUGGINS. 

 The narrower and more sharply defined calcium line K, in itself distinguishes the 

 division from those which precede and follow it. The delicate absorption lines, 

 which also distinguish the type, are difficult to photograph, but they are well shown 

 in the spectra of Sirius and Vega. The spectrum of a Cygni has been sometimes 

 classed differently, but a comparison with the spectrum of Sirius, which -is placed 

 next it, clearly shows their identity, and that they only differ in strength. The fine 

 lines appear to be due to calcium and titanium rather than to iron, although that 

 spectrum is also present in an incipient form. The distribution of this division is 

 irregular, viz. : 



A. B. C. D. AA. 



10 7 3 3 



Division III., or tl.se hydrogen-iron type, is the last of the separate divisions into which 

 Type I. has been divided. In its more advanced examples the iron spectrum is fully 

 developed. That spectrum has been plotted on the scale for comparison. This type 

 is more closely allied to the subsequent solar type, Division IV., than to the preceding 

 hydrogen and helium types. The brightness of the violet end, and the obscurity of 

 the red end of the spectrum, remain the same as in the preceding divisions. The 

 hydrogen lines remain very strong, and the calcium lines K and H are generally 

 subordinate to them. 



This division completes the requisite subdivision of SsccHi'sType I., which has not 

 hitherto been fully established. The subsequent divisions remain as defined by 

 SECCHI in 1868. It must also be remembered that SECCHI fully recognised the special 

 character of the spectra of the Orion stars. 



Division IV. is equivalent to Type II., or the solar type. The characteristics of this 

 type are well defined. They are also elucidated by our more intimate knowledge of 

 the solar spectrum, which forms the basis of our knowledge of astronomical physics. 



Division V. is equivalent to Type III., and is the first of the banded types 

 investigated by Duner. The photographs of this type of spectra are difficult to 

 take. These stars are not numerous within the range of magnitude under con- 



