STARS IN COMA BERENICES. 399 



tion of the stars. Then, again, the number of stars visible on 

 the different plates varies greatly. This is the case even when 

 the exposures were taken on the same night, although these 

 were always of the same length, namely six minutes. The larg- 

 est number of stars is found on the plates taken in 1875. In 

 spite of their variable quality, however, I decided not to omit 

 any of them, but to measure on each one all the stars that 

 could be plainly seen. This was necessitated by the fact that 

 I had but three plates of the early date ; and again but five 

 taken in 1875 and showing a fairly large number of stars. 

 None of these could well be rejected without seriously injur- 

 ing either the proper motions in the one case, or the positions 

 in the other. But thereby the standard of excellence was 

 placed so low, that none of the others could legitimately be 

 omitted. The result is, that some of the stars show quite large 

 residuals, due' to the difficulty of measuring hazy images. Es- 

 pecially is this the case for stars just on the limits of visibility, 

 and for those lying near the edge of the plate, where radial dis- 

 tortion becomes very marked. On the whole, the cluster is 

 not well adapted to photographic measurement, as it is very 

 scattered, and the range of brightness is large. 



The origin of coordinates was taken to coincide with star 14 

 (i2e Comae Berenices). For the reduction it is necessary that 

 the point be known approximately where a line from the optical 

 center of the lens strikes the plate perpendicularly. Ruther- 

 furd always so adjusted his instrument, that this line should 

 pass through the image of some bright star, no. 14, in my case. 



I subjoin Table I, giving all necessary data regarding the 

 plates. The column " Date " shows the date, and that headed 

 " Sidereal Time," the time of exposure. This latter is the mean 

 of four instants, namely the beginning and end of the first, and 

 the beginning and end of the second exposure. Next follow 

 the reading of the barometer, together with the attached and 

 external thermometers. The 7th and 8th columns refer to 

 the telescope, the former showing the readings of a thermo- 

 meter in contact with the tube, and the latter the reading of a 

 micrometer head at the eye end. This latter quantity depends 



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