Researches in Stellar Photography. 



211 



relative brightness, only after varying times of exposure. It would 

 therefore have been premature, or even unscientific, to argue a priori 

 ■of the reliability of the latter measures from those of the former. 



From inspection of Table V it will be found that the general error 

 of the whole group of measures of the photographic plates, re- 

 garded as measures of the same quantity, is 0*24", and of the helio- 

 meter 0"29". Even this minute error would probably have been 

 smaller had it not been for a practical difficulty, which arises in 

 the accurate bisection of the disks of the star images. This diffi- 

 culty, though it exists also to a proportionate extent in the use of any 

 filar micrometer applied to the brighter stars in the skies, does not 

 exist to an equal degree in the case of heliometer measures. Never- 

 theless, such is the convenience of leisurely bisecting even the com- 

 paratively large photographic disk on a steady film, that the final 

 results of the photographic measures are, on the whole, slightly more 

 accordant than those secured by the use of the heliometer. In point 

 of facility and rapidity, the advantages of the photographic method 

 are enormous. 



Having mentioned the error necessarily connected with bisection 

 of the star disk, it is desirable to present a specimen of what 

 actually occurs. Accordingly I give a short table, containing the 

 detail of the errors made in continually bisecting two star disks, such 

 :as actually occurred in reference to Maia and Asterope, of diameters 

 25" and 15" respectively. 



Table VII. 



Maia, mag. 4 "00. 



Asterope, mag. 5 "98. 



Reading of screw 

 at apparent 

 bisection. 



Difference 

 of readings from 

 mean in 

 seconds of arc. 



Reading of screw 

 at apparent 

 bisection. 



Difference 

 of readings from 

 mean in 

 seconds of arc. 



in. 

 -90822 

 '90811 

 -90826 

 0-90841 

 -90859 

 -90861 

 0-90870 

 -90846 

 0-90827 

 0-90810 

 -90816 

 0-90811 

 0-90816 



// 



0-17 

 0-36 



o-io 



0-15 

 0-46 

 0-50 

 0-65 

 0-24 

 0-09 

 0-38 

 0-27 

 0-36 

 0-27 



in. 

 -90774 

 -90772 

 -90793 

 -90799 

 -90785 

 -90788 

 -90803 

 -90791 

 -90788 

 0-90781 

 -90779 

 -90799 



o'-25 

 0-28 

 0-09 

 19 

 0-05 

 0-00 

 0-26 

 0-05 



o-oo 



0-12 

 0-16 

 0-19 



Mean '90832 



31 



0-90788 



0-14 



