568 



SCIENCE 



[N. S. Vol. XXV. No. 641 



lines was begun, with a view to determin- 

 ing whether there is a shift of any lines 

 which is progressive from spectral type to 

 type. Several lines were found to undergo 

 such a progressive change, as is indicated 

 by the radial velocities obtained from them. 

 An examination of Rowland's tables shows 

 that in most cases, lines apparently single 

 are in reality blends of two or more close 

 components. The nature of the variations 

 is such as to indicate varying intensities of 

 the same components rather than the pres- 

 ence or absence of different components in 

 the different types. The investigation was 

 limited to types F to Mh inclusive on the 

 Harvard classification. In this classifica- 

 tion the sun is of type G. 



A comparison with Adams's list of sun- 

 spot lines gives strong indications that the 

 physical conditions in the stars as we pass 

 from the F to the Mh type vary in the 

 same direction as from the sun to the sun- 

 spots. 



It was thought possible that for variable 

 stars of large light changes, traces of ve- 

 locity variations for some of the lines 

 might be found, corresponding to small 

 changes in spectral type as the stars varied 

 from maxima to minima— -and vice versa. 

 In the case of o Ceti actual changes in the 

 character of the spectrum are well estab- 

 lished, though up to the present no appre- 

 ciable changes in the wave-lengths of any 

 of its spectrum lines have been obsei-ved — 

 leaving out of account the large displace- 

 ments of the bright hydrogen lines. A 

 comparison of the available measures of 

 rj Aquilce, a variable star of the fourth 

 class with a range of only 0.8 of a magni- 

 tude in light variations, showed evidences 

 of variations in the positions of some lines 

 from light maximum to minimum similar 

 to the variations that were found from 

 type to type. A further study of this 

 variable star is desirable to establish 



definitely the exact character and amount 

 of these variations. 



These variations in the wave-lengths of 

 some of the lines depending on spectral 

 type, will make necessary the exercise of 

 great care in the selection of lines in radial 

 velocity determinations, and a proper 

 allowance for the type. 



A Device of eliminating Guiding Error 

 from Photographic Determinations of 

 Stellar Parallax: Frank Schlesinger. 

 Experience has shown that a fruitful 

 source of error in photographic work is 

 what has been aptly termed the 'guiding 

 error.' This arises from the fact that 

 when the guiding is imperfect, the image 

 of each star wanders from its mean posi- 

 tion and these excursions are registered 

 upon the photographic film to a different 

 extent for stars of different brightness. 

 This error is particularly troublesome in 

 parallax work where the star under exam- 

 ination is usually much brighter than the 

 comparison stars. After considerable ex- 

 perimenting the writer adopted the follow- 

 ing simple device that seems to overcome 

 the difficulty in a very satisfactory way. 

 A rotating disc was mounted a little below 

 the center of the photographic plate and 

 about ten millimeters in front of it; that 

 is, on the side toward the objective. The 

 disc was made in two halves moving on the 

 same axis, so that the opening left between 

 their edges could be adjusted to any de- 

 sired angle. The image of the bright star 

 having been brought to the center of the 

 plate and the disc set into motion, a part 

 of the star's light will be occulted in each 

 rotation of the disc. The amount of light 

 that reaches the plate depends only upon 

 the angular opening between the two halves 

 of the slit and neither upon the rate at 

 which they are revolving nor the distance 

 of the image from the center of rotation. 

 The images obtained in this way were free 



