194 Prsesepe Group; Measurement and Reduction 



measured on each da} 7 , and as the photographs of Prsesepe show 

 about forty-five stars that admit of measurement, it was neces- 

 sary to spend two days on each position of the plate. To 

 eliminate the effect of a possible motion of the plate or of the 

 scale the central star 15 was read each day by both observers. 

 After three of the plates had been measured, viz. : III, VII and 

 IX, it was decided to read the central star more often, so that on 

 the succeeding plates four such readings were made every day, in- 

 stead of two. 



Three observers were engaged in the measurement of the first 

 five plates, but only two were concerned in the work for any single 

 day. Care was always taken to have the same pair of observers 

 make both the direct and the reversed measurements of a particu- 

 lar set of stars, in order to eliminate personality. Suppose one 

 of the observers has contracted the habit of always setting the 

 micrometer wire too far to the right of the centre of a star's im- 

 age by an amount depending upon the size of the image ; the 

 distance between two stars as obtained hy such an observer will 

 be subject to an error which depends upon the difference of magni- 

 tudes of the stars. But when the plate is reversed 180°, the same 

 observer will get a distance which is too small by as much as the 

 first distance was too large or vice versa ; consequently the mean 

 of the two measurements will be free from such personality as we 

 have supposed. However, this method of measurement does not 

 eliminate all personality, for the star images are seldom round 

 and are usually more sharply defined on one edge than upon the 

 other; two observers will thus sometimes differ considerably in 

 their estimations of the true centre of the image. 



Table II gives the runs, circle reading, etc., for each daj\ Runs 

 were observed twice daily, once before and once after the measure- 

 ment of the stars ; the number in the column headed " runs " is the 

 mean of the two determinations expressed in millimetres. The 

 circle was also read twice, employing two microscopes 180° apart 

 for each reading ; in the column marked " circle " the degrees and 

 minutes are always taken from the right-hand microscope, while 

 the number of seconds is the mean of both microscopes. The 

 thermometer occupied a fixed position near the plate and was 

 graduated in Fahrenheit degrees. The last column gives the in- 

 itials of the three observers, Kretz, Hays and Schlesinger. 



