240 Rutherfurd Photographic Measures. 



importance when the reduction of the other clusters is undertaken. 

 The details in regard to them will be found under Y. in the present 

 paper. In general, an effort has been made to let the reduction 

 of each plate be separate from that of the others as far as possible. 

 General correction tables have in most cases been replaced by special 

 tables for each plate. Thus the numbers obtained control each other, 

 and the results from each plate become available before all the plates 

 are finished. 



In making the exposures, two impressions of the cluster were 

 taken on each plate. The second impression is West of the first. 

 After it was finished, the clock-work of the telescope was stopped, 

 and the cluster allowed to move off the plate, the brighter stars 

 leaving " trails," or black lines on the developed negative. Shortly 

 before the central star reached the edge of the plate, the clock was 

 again started, and the central star was allowed to impress itself 

 once more. This final image of the central star gives us the orient- 

 ation of the plate, as after the application of the proper corrections, 

 it furnishes the direction of position angle 270 . (See YI.) Both 

 the Eastern and Western impressions were always measured. In 

 the reductions these sets of measures have been treated as if they 

 came from separate plates, a proceeding which helps to eliminate 

 errors of computation. Each complete measure of position angle 

 comprises three settings on the last image of the central star, and 

 three on the star to be measured, in both positions of the measuring 

 micrometer ; or twelve settings in all. The measures of distance 

 consist of similar means of twenty settings. There are in all 1262 

 complete measures of distance, and as many of position angle. 

 The following table gives various quantities connected with the 

 several plates. The first column contains the number of the plate, 

 those originally marked on the plates after development being re- 

 tained. The second column contains the date ; the third the middle 

 sidereal time of the exposure. The column headed Tel. Therm. 

 gives the reading of a thermometer attached to the tube of the tele- 

 scope. The column focus gives the reading of a micrometer head, 

 which measured the position of the plate-holder. In the column 

 marked zero is given the distance of the last (or orientation) im- 

 pression of the central star from the origin of co-ordinates. These 

 distances are expressed in divisions of the glass scale of the measur- 

 ing micrometer. The last columns give the zenith distance (£), the 

 parallactic angle (q), and the refraction constant (*), for the central 

 star 24 p. 



