1921] on Advances in Astronomy 355 



must be as perfect as possible. (2) The lenses of large object-glasses 

 must be adjusted with great care so that there may be neither tilt 

 nor eccentricity between them. (3) Photographs should all be taken 

 with the telescope pointing in the same direction. One cannot be 

 taken when the field is east and another when it is west. Atmospheric 

 dispersion and possibly minute flexure of the lenses cause slight 

 deformation of the images which may be scarcely visible to the eye, 

 but appear in measures. (4) The star the parallax of which is being 

 determined and the comparison stars should have approximately 

 equal images on the photograph. This is secured by means of a 

 rotating shutter, a neutral screen, or the use of a grating in front of 

 the objective. 



The purpose of (3) and (4) is to make any residual errors the 

 same for the parallax star and the comparison stars, and so far as 

 possible the same on all photographs. 



The knowledge of the distance of a star gives us immediately its 

 luminosity or the amount of light it emits as compared with the sun. 

 There is a very great range in luminosity even for stars of the same 

 spectral type. Now the stars have been arranged in an order according 

 to the spectra, which agrees fairly well with their order in colour 

 from blue to red, and is essentially an arrangement according to 

 temperature. This may be regarded as an extremely good first 

 approximation to a classification of stellar spectra. But it does not 

 detect any differences attributable to absolute luminosity, though 

 presumably density and gravity at the surface layer of the star from 

 which the lines in the spectrum have their origin must be- widely 

 different. 



A few years ago a very fruitful investigation was commenced at 

 Mount Wilson by Adams and Kohlschuter. By a close comparison 

 of the spectra of stars of the same spectral class, but differing greatly 

 in absolute luminosity, they detected lines the intensities of which 

 differ. Adams and his coadjutors at Mount Wilson have pursued 

 this research with very great success. They have found in stellar 

 spectra a number of pairs of neighbouring lines, one line of each pair 

 being independent of the absolute luminosity, while the other changes 

 in intensity with the luminosity of the star. They have measured 

 the relative intensities of these pairs of lines, and compared their 

 measures with the luminosities of 650 stars already known through 

 the trigonometrical determinations of parallax made at Allegheny, 

 McCormick, Mount Wilson, and Yerkes. Thus they have found the 

 luminosities of stars corresponding to different intensities of the 

 lines. They have recently published a catalogue (Astrophysical 

 Journal, Jan., 1921, Vol. LIII. p. 13) giving the luminosities and 

 parallaxes of 1680 stars. 



The advantage of this method is that it extends the range of 

 parallax determinations beyond the limit (say) 0'02" of the 

 trigonometrical method, the limit of the spectroscopic method being 



