284 



Praesepe Group ; Measurement and Reduction 



Scale-Value. 



Plate. 



Cor. for 



Cor. 



Corrected 



Tel. Therm. 







Aberration. 



for Temp. 



Scale-Value. 







I 



— O.OOOO99 



O.OOOOOO 



52.8701 



+58° 



8.4 



II 



— 99 



+ 2 



52.8715 



58 



8.4 



III 



— IOO 



— 3 



52.8712 



53 



8.4 



IV 



— IOO 



+ 4 



52.8760 



53 



8.4 



V 



- 98 



+ 3 



52.8788 



48 



7-8 



VII 



— 99 



— 2 



52.8827 



58 



7-7 



VIII 



99 



— 5 



52.8831 



58 



7-7 



II 



- 98 



— 2 



52.8840 



48 



7.8 



The mean of the scale-values is 



52".8772 



and if we adopt the correction of -f 0.000009 as indicated by 

 comparison with the heliometer places, this becomes 



52 /y .8776. 



However, either of these must still be regarded as only an ap- 

 proximate value, since the separate values for the different plates, 

 as given above, vary in a way that cannot be fully explained by a 

 connection with the readings either of the telescope thermometer 

 or of the " focus." 



The above investigations on the orientation and on the scale- 

 value lead to the same conclusion ; it will usually be better to de- 

 termine all the constants of a plate by comparing the measures of 

 some of the stars with their positions as known through meridian 

 observations or otherwise, than to attempt to reduce them by 

 means of a predetermined scale-Talue and orientation. In any 

 case it is necessary to appeal to such known positions to deter- 

 mine the values of k and c, or the absolute place of the group in 

 the sky. The positions of two stars are theoretically sufficient to 

 determine all four constants, but in most cases it will be possible 

 to find enough stars to eliminate errors of observation to a large 

 extent. 



In conclusion, I wish to acknowledge my indebtedness to 

 Messrs. Kretz and Hays for assisting me in the measurement of 

 the plates, and to Professor Jacoby, who has kindly explained to 

 me the methods used by him in the measurement and reduction 

 of stellar photographs, and who has also suggested some improve- 



