PROGRESS IN ASTROPHYSICS ABBOT. 185 



observations at Yale was 0.39" for the faint star known as Lalande 

 21185. This star is invisible to the eye, being of magnitude 7.3 

 and has a proper motion of 4.77" per century. 



Another method of parallax investigation which has been devel- 

 oped in recent years to a high state of perfection is that by photog- 

 raphy. If a photograph of a celestial region containing the star 

 whose parallax is to be determined is made in the earlier part of the 

 night at one epoch, and again in the latter part of the night at an 

 epoch six months later, the position of the parallax star will in gen- 

 eral be found to be changed with respect to the mean position of all 

 the fainter stars in its neighborhood. After clearing the apparent 

 motions for tlie known proper motions of the stars in question, a 

 residual effect will be left, due to the fact that the parallax star is 

 in general nearer than the fainter stars in the background. In a 

 method proposed by Kapteyn the photography was done in the fol- 

 lowing manner: A plate was exposed at a certain epoch, then kept 

 without developing for six months, exposed again in a slightly dif- 

 ferent position, and then kept still another six months, and finally 

 exposed a third time before developing. Thus three series of images 

 of all the stars would be found upon the plate, of which two would 

 be taken Avith the earth in one part of its orbit and the other with the 

 earth at the opposite part. Recently Prof. Schlesinger, formerly of 

 the Yerkes Observatory, now director of the Allegheny Observatory, 

 has used the photographic method with the great Yerkes refractor, 

 and has obtained parallaxes for about 25 stars of a very high order of 

 accuracy. Prof. H. N. Russell, of Princeton, also has obtained excel- 

 lent results by this method for 52 stars observed by himself and 

 Hinks at Cambridge, England. These observers did not leave the 

 plates undeveloped for a year or 18 months, according to the method 

 proposed by Kapteyn. but preferred to take separate plates at the 

 different epochs. This parallax work by photography is becoming 

 extremely well thought of by astronomers, and is engaging more and 

 more the efforts of those who have large refracting telescopes avail- 

 able for this purpose. 



It is found, as would be expected, that in general the brightest 

 stars are nearer the earth, and the stars whose proper motions are 

 largest are also nearer the earth. Prof. Kapteyn published, in 1902. 

 a formula connecting the quantities parallax, stellar magnitude, and 

 proper motion. This is found to agree pretty well with more recent 

 work. Prof. Lewis Boss, in his interesting discussion of the great 

 Preliminary General Catalogue of positions and proper motions 

 of stars, recently published by the Carnegie Institution, has also 

 derived a formula connecting the proper motion and the parallax for 

 stars of the 5.3 magnitude. 



