31 



377 



The colours of the 236 stars for which m,„, > 1 l-^-QS and p (/;) > 99-5 m'^ 

 are distributed as follows 



limits of /; 



number of stars . . 



•0 -1 -2 -3 -4 -5 -6 -7 



1 T 36 34- 22- IS" 19- 



•7 -8 -9 rO ri 1-2 1-3 1-4 1-5 I'S Vl 



10 13- 21- 19-9 7 6-2 6 V 



In Figure 4 the full curve represents the smoothed relative frequencies of /; 

 for these faint stars of mean photographic magnitude 13"' forming the background 

 of the Pleiades cluster. For comparison the corresponding curves are shown firstlj' 

 for the 103 stars in the whole skj' which are brighter than 3"' photographicallj^ 

 according to King (Harv. Ann. 71, 21 and 76, 117), broken line, and secondly for 





Figure 4. 



the 136 stars north of the equator and brighter than 4'" photographicallj' (Leiden 

 Ann. 14, II), dotted line. The bad agreement between these two additional curves 

 may be mainly due to unreliability of the material used. At anj' rate the difference 

 in the colour distribution shown bj' stars of nipg =13™ in the Pleiades and of 

 jTipg = 3™ taken from the whole sky is verj' striking. This difference may, as is 

 well known, be explained without assumption of selective extinction of light in 

 space, simply by the falling off in star density with increasing distance from our 

 sun thus causing a lack of absolutely bright and white stars among the apparently 

 faint stars. 



16. Double and Neighbouring Stars. 



In Table 15 the 33 more conspicuous pairs of stars, noted during measure- 

 ment of the plates, have been listed. The narrowest of these pairs (no^ 408 and 

 407) shows a distance of 1"'5 between the two components, while among the 

 following double stars, which were not recognized as double on the plates, is one 

 of about 1"'6 separation. 



