Permanence of the Rutherfurd Photographic Plates. 265 



7. We take up next the instrumental corrections which must be 

 applied to the observations, so as to obtain the true rectangular 

 coordinates of the stars on the plate. These corrections are two 

 in number, the angle correction and the scale correction. The 

 angle correction is needed because it was not possible to turn the 

 plate through an angle of exactly 90° when passing from the 

 measurement of one coordinate to that of the other. The angle 

 through which the plate was turned could always be made very 

 nearly go°, and its amount could also be measured with very high 

 precision, but it was impossible to make it exactl}^ 90°, within 

 the limits of measurement by the position circle microscopes. A 

 correction is, therefore, needed to make the measured coordinates 

 truly rectangular with respect to each other. 



The scale correction has been already mentioned. It is simply 

 a correction proportional to the measured coordinate, of such a 

 magnitude as to make the difference of coordinates of the standard 

 stars constant on all days when measures of the coordinate in 

 question were made. 



The circle readings were reduced in the following way : The 

 number of separate positions in which coordinates were mea- 

 sured on each plate was eight: These were as follows : 



1. a;-coordinate. 



2. ^-coordinate. 



3. a;-coordinate, reversed. 



4. y-coordinate, reversed. 



5. 6, 7, 8. The same in the special standard star measures, 

 made for the purpose of securing uniformity of scale value in both 

 coordinates. 



As an example of the angle measures, let us take those for plate 

 16, which were as follows, each number being, of course, the mean 

 of all the readings taken in one position of the plate. 



1. 182° 26' 21" a;-coordinate. 



2. 92° 26' 22" ^/-coordinate. 



3. 2° 26' 20" a;-coordinate, reversed. 



4. 272° 26' 20" ^/-coordinate, reversed. 



5. 182° 26' 21" a;-coordinate, special standard star measures. 



6. 92° 26' 20'' y-coordinate, special measures. 



7. 2° 26' 20" a;-coordinate, special measures, reversed. 



8. 272° 26' 20" y-coordinate, special measures, reversed. 

 Mean 26' 20" 



