198 Permanence of the Rutherfurd Photographic Plates. 



microscope allows both coordinates to be measured in one posi 

 tion of the microscope b}^ referring the star to the nearest straight 

 lines. In the present research, only the horizontal screw of the 

 microscope has been used. For the Rutherfurd plates have not 

 the photographed reseau, and therefore the measures were made 

 by the method described above in which the two coordinates are 

 separately obtained by means of a direct comparison with the 

 scale. 



The errors of the measuring machine which can have an in- 

 fluence upon the measured coordinates, and which were therefore 

 investigated with considerable care, are as follows : 



1. Errors of the micrometer screw. 



2. Division errors of the scale. 



3. Deviations from perfect straightness of the guiding cjdinder 

 of the machine. 



4. So-called errors of projection. These errors are caused by 

 the fact that the trunnions about which the microscope revolves 

 do not maintain a perfectly constant direction when the micro- 

 scope with its trunnions is moved to different positions on its 

 slider. This is caused by a lack of perfect straightness in the 

 slider. The consequence of any slight change in the direction 

 of the line joining the trunnions is of course to produce a slight 

 error when the microscope is revolved about the trunnions in the 

 comparison of the star with the scale. The existence of small 

 errors from this source was pointed out by Donner (Acta 

 Soc. Fenn. XXI. No. 8). Prof. Donner gave a method for the in- 

 vestigation of these errors. I have, however, preferred to neglect 

 them altogether in the present research for the following reasons. 

 Their investigation is a somewhat laborious operation requiring 

 a great deal of time. Moreover, it is almost impossible to obtain 

 a determination of them which will have quite sufficient weight. 

 For this reason, the Messrs. Repsold have devoted considerable 

 attention to devising a form of machine which will not be subject 

 to these errors at all. They have come upon a plan which prom- 

 ises complete success, and this plan will be carried out in the new 

 machine which they are now constructing for the observatory of 

 Columbia University. It follows that if the errors in question 

 had been elaborately investigated for the purpose of the present 

 research, the results would not have been useful any further, be- 

 cause the next series of measures undertaken will be made with 



