200 Permanence of the Rutherfurd Photographic Plates. 



Consequently the progressive errors have been determined for the 

 points : 



E R K K E E 



5.0, 7.0, 9.0, ii.o, 13.0, 15.0. 



The errors determined for each point are independent of any as- 

 sumed law for the variation of the progressive errors of the 

 screw. The portion of the scale used for the present purpose is 

 numbered from 73 to 78 inclusive, the numbering on the scale de- 

 creasing with increasing readings of the micrometer screw. The 

 observations were made in a series of operations in each of which 

 the screw was read on all the divisions of the scale, between 73 and 

 78, possible in the position occupied by the microscope during 

 that operation. And the position of the microscope was changed 

 between the operations in such a way that the observations in 

 the successive operations were obtained in accordance with the 

 following scheme. This scheme shows the results of two separate 

 series of observations made by Mrs. Herman S. Davis. 



Operation 1. 



First Series. 

 Scale. Screw. Diff. 



Second Series. 

 Scale. Screw. Diff. 



78 13.0014 



77 15.0018 2.0004 



78 



77 



13.0100 

 15.0109 



2.0009 



Scale. 



Screw. 



Operation 2. 



First Series. 



Diff. 



Second Series. 



Scale. 



Screw. 



Diff. 



78 

 77 

 76 



11.0020 

 13.0040 

 15.0058 



2.0020 

 2.0018 



78 II. 0156 



77 13.0220 



76 15.0200 



2.0064 

 1.9980 



Scale. 



Screw. 



Operation 3. 



First Series. 



Diff. 



Second Series. 



Scale. 



Screw. 



Diff. 



78 

 77 

 76 



75 



9.0025 



11.0070 2.C045 



13.0100 2.0030 



15.0119 2.0019 



78 

 77 

 76 



75 



9.0045 

 11.0080 



13-0115 

 15.0150 



2.0035 

 2.0035 

 2.0035 



