VII. 



Orientation by Trails. Scale- Value. 



An independent method for orienting a stellar photograph is 

 furnished by the " trails " or third images of some of the brighter 

 stars. The Rutherfurd photographs previously reduced depend 

 upon this mode of orientation, and the present research offers an 

 admirable opportunity for testing its accuracy. Four trails have 

 been measured and reduced on each Prsesepe plate, and the re- 

 sulting values of the orientation corrections were compared with 

 the results obtained from a comparison with meridian observa- 

 tions, and also with those obtained with the use of the heliometer 

 places. On Plate II the trails were too faint to admit of measure- 

 ment, and on Plate V they were missing altogether. 



The trails were measured in a different manner from that used 

 for the other images. The plate was first set in the position 

 which it occupied when " y direct " had been measured for the 

 stars, and the micrometer was set and read on the east image of 

 a star whose trail was to be measured. Then, without touching 

 the microscope, the plate was moved along the cylinder till the 

 corresponding trail came into view. This was alwaj^s possible 

 because the plate had been approximately oriented when first set 

 in the machine. Two readings were made upon the trail and the 

 plate was then moved back to the east image, which was read a 

 second time. The same operations were gone through for the 

 west image, and the mean of all the readings on the images was 

 subtracted from the mean of the readings on the trail, thus giving 

 the offset in declination by which the trail differed from the 

 middle point between the two images. All the above operations 

 were repeated in the opposite position of the plate, namely that 

 corresponding to " y reversed," except that in the latter case the 

 mean of the readings on the trail was subtracted from that for the 

 images, so as to get the same sign for the offset as before. Each 

 trail was thus measured by two observers separately, so that in 

 all, sixteen readings were made on each trail, and eight upon each 

 of the images. The resulting offsets are tabulated below in mil- 

 limetres. 



279 



