1. Introduction. Instrumental Constants. 



L he present investigation has mainly been undertaken from the following 

 point of view. 



The Pleiades have always been the object of special attention. As a conse- 

 quence of this a number of old photographs with long exposures of this group is 

 available. These plates have partly been taken with lenses of sufficient focal length 

 to enable a good determination of relative proper motions of faint stars bj' com- 

 parison with more recent plates. Owing to the considerable number of such old 

 photographs the attainable accuracy will perhaps be greater for this group than for 

 any other region of the sky. 



It is to be regretted that the determination of absolute proper motions of the 

 fainter stars by connection with the brighter ones of known proper motions is 

 made difficult by the unknown magnitude equation occurring in photographic work 

 of this kind. But just in the group of the Pleiades we may hope through accurate 

 measures to find out, which stars, also among the fainter ones, have a proper 

 motion common to the system and which have not. Of course, a certain arbitrari- 

 ness will alwaj's remain in the separation of these two groups. Determinations of 

 radial velocity would help to distinguish physical members from stars only optic- 

 ally projected on the part of the sky considered. We should then be able to use 

 the stars of supposed common proper motion but of different brightness for the 

 elimination of the magnitude equation. 



Once the separation between physical and only optical members of the group 

 is made, the determination of colour equivalents of the same stars will give us: 

 Firstly, the relation between absolute magnitude and colour of the members of this 

 interesting sj'stem, and secondly, the relation between apparent magnitude, colour 

 and proper motion of faint stars in a special well observed region of the sky. 

 Therefore I included the Pleiades in my programme for the determination of effective 

 wavelengths with the 60 inch reflector of Mount \Yilson. I had the prospect, in 

 this way to get colour-equivalents of as faint stars as occur on the old photographs 

 fit for the determination of relative proper motions. 



The conspicuous group of the Pleiades has a diameter of about 2°, while the 

 60 inch reflector using the full aperture has a field of only about 20' diameter fit 



40* 



