SPECTRA OF STARS TO THE 3 MAGNITUDE. 131 



sideration. The spectra are closely allied to those of Division IV., and have been 

 placed consecutively with them. The coincidences with the spectra of calcium and 

 manganese have been marked on the scale. 



Division VI. is equivalent to Type IV. There are no stars in this division 

 brighter than the 5|- magnitude, but to complete the series, photographs of 

 the spectra of SECCHI'S Superba, and of 19 Piscium have been included. The 

 photographs are poor owing to the faintness of the stars, but their interest lies in 

 the hydrocarbon absorption bands, and further in the line spectrum, also recognizable, 

 and similar to that of a Tauri (Aldebaran). 



There remains the question of how far the distribution of the stars into the eifht 

 equal areas discloses any information as to the distribution of their respective types 

 of spectra in space. 



The dimensions of the sphere enclosing stars to the 3^ magnitude is obtained from 

 the light ratio. The decrease of light for each magnitude beyond the 1st is inversely 

 as the light ratio 2 '51. The corresponding increase of distance is as the square root 

 of this ratio, or as 1 to 1*58. Taking the distances of the 3f magnitude stars arrived 

 at in this way as the radius of the sphere containing the stars of the 3^ magnitude, 

 its radius will be, in terms of the mean distance of 1st magnitude stars from the sun, 



= 1 X (l'58) v = 3'5 approximately. 



The mean distance of 1st magnitude stars from the sun has been determined from 

 parallax observations to be approximately 36^ light years. Thus the diameter of the 

 enclosing sphere for stars of the 3^ magnitude is approximately 255 light years. 



This gauge block of space, although large, might be so small, compared with the 

 dimensions of the galaxy, as to disclose nothing with regard to its structure. The 

 following table of distribution, however, shows that within this block there are 

 indications of differences in the distribution of the different stellar types, relatively 

 to the plane of the galaxy. The table shows the distribution of the gaseous nebulte, 

 both planetary and extended, for the same areas. 



