190 



Prsesepe Group; Measurement and Reduction 



Table I gives the data of exposure for the plates. Plates YI,X 

 and XI do not appear, these being the ones that were not meas- 

 ured, on account of their inferiority. This is due to the fact that 

 the photographic images of the stars on these plates, when viewed 

 under the microscope of the measuring machine were neither 

 so round nor so well defined as on the other plates and therefore 

 did not admit of so accurate measurement. The irregularity of 

 the images is not due to a deterioration of the plates since they 

 were taken, but to the bad behavior of Rtjtherfurd's clock dur- 

 ing the exposures. For this reason these three plates were never 

 measured, it being deemed probable that more reliable results are 

 to be obtained from the eight plates actually reduced, than if all 

 the plates had been measured, in spite of the greater number of 

 observations in the latter case. In this connection I should also 

 sajr that not all the stars which appear on the plates were meas- 

 ured. A few whose images come near the edges of the plates 

 were rejected, for not only are these images much distorted, but 

 as we shall see later, the corrections become uncertain as we re- 

 cede from the centre of the plate. 



Table I. — Photographs op Prsesepe. 



Lat. 



Observatory of L. M. Rutherfurd, New York. 

 4o°43'48".5 Long. = 4 h 55 m 56 s .62 W. 



No. 



Date. 



Sidereal Time. 



Bar. 



Thermometers. 



Focus 











Att. 



Ext. 



Tel. 





I 



1870 Apr. 24 



h m s 

 10 45 05 



30.01 





 60 





 55 





 58 



8.4 



II 



1870 Apr. 24 



II 25 35 



30.01 



60 



55 



58 



8.4 



III 



1870 Apr. 25 



II 10 35 



30.26 



53 



47 



53 



8.4 



IV 



1870 Apr. 25 



11 59 35 



30.26 



53 



47 



53 



8.4 



V 



1877 Apr. 14 



10 39 38 



30.06 



47 



45 



48 



7.8 



VII 



1877 Apr. 25 



11 26 02 



30.06 



57 



56 



58 



7-7 



VIII 



1877 Apr. 25 



11 53 32 



30.06 



57 



56 



58 



7-7 



IX 



1877 May 2 



10 57 08 



29.86 



47 



46 



48 



7.8 



The column marked " sidereal time " gives the mean of four in- 

 stants for each plate : beginning of east exposure, end of east ex- 

 posure, beginning of west exposure and end of west exposure ; 

 each exposure lasted six minutes. Three thermometers were 

 read : attached, external and telescope, the last being in contact 



