II. 



Measurement of the Plates. 



The plates were measured with the older Repsold Measuring Ma- 

 chine of this Observatory, which is a counterpart of the one by the 

 same maker belonging to the University of Leyden except that an 

 alteration has been made which obviates " projection errors." 

 (See III.) A full description of the Leyden machine is given 

 in the "Bulletin du Comite Permanent," Yol. 1, page 169, and 

 also in the recent work by Dr. Scheiner, " Photographie der Ge- 

 stirne," page 148. The machine is so constructed that the posi- 

 tion of a star may be determined either by position angle and 

 distance or by rectangular coordinates ; the latter method was 

 adopted in the present case. A star which is to be measured 

 may be brought into the field of the reading microscope by 

 moving the plate along a straight guiding cylinder and then 

 moving the microscope at right angles to the cylinder on another 

 straight guiding way. The wire of the micrometer is made to bi- 

 sect the image of the star and the micrometer head is read. Then 

 the whole microscope is revolved through a small vertical angle 

 and the wire set upon a scale of millimetres placed parallel to the 

 motion of the microscope. The difference of the two readings 

 on star and scale, together with the number of the line on the 

 scale gives us the position of the star. Having gone through 

 the same operation for all the stars we obtain their relative 

 positions, at least in one direction ; the plate is now revolved 

 through 90° by means of the graduated circle and the stars are 

 again measured ; these two sets of measures are sufficient to fix 

 the relative positions of the stars, but in order to secure greater 

 accuracy and especially to eliminate personality the plate is turned 

 180° and 270° respectively from its original position, and the 

 stars are read a third and a fourth time. By means of the trails 



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