Rutherfurd PJwtographic Measures. 



The residuals are not quite satisfactory. I have therefore discussed 

 the Pleiades scale values by means of formula (a), omitting the 

 discordant plates 24 and 25, and find the temperature coefficient: 



o .000230 



o .000040. 



It is further to be remarked, that the residuals from formula (a) are 

 somewhat less than we should g.et if we left the temperature and 

 focal reading altogether out of consideration. 



To complete this part of the subject, it is necessary to examine 

 the evidence of the ^ Cygni plates.* For this purpose I selected 

 from the Rutherfurd p Cygni list four pairs of stars, suitably 

 di.stributed in position angle. Treating the data exactly as already 

 described for the fi Cassiopeite plates, gives the temperature coeffi- 

 cient : 



— 0^.00000696 db 0^.00000167, or — o".oooi95 ^ o".oooo47 . 



The low weight of this result, like that from the Pleiades series, 

 is of course due to the small number of plates used, and to the 

 comparatively slight range of temperature. Assembling the three 

 values obtained, w^e have for the temperature coefficient : 



fi Cassiopeife 

 /3 Cygni . . . 

 Pleiades . . . 

 Mean by w^eight 



. — 0.000119 zfc 0.000008 



. ■ — 0.000195 ^ 0.000047 



. — 0.000230 ± o 000040 



. — 0.000125 ^ 0.000008 



The evidence of the j3 Cygni and Pleiades plates practically does 

 not change the result from the ft Cassiopeiae plates. I am therefore 



* Ann. N. Y. Acad, of Sci., vol. vi, pp. 331, et 



seq. 



