F. L. O. Wadsworth — Determining the Eccentricity, etc. 373 



Art. XLII. — A Simple Method of Determining the Eccen- 

 tricity of a Graduated Circle with one Vernier / by F. L. O. 

 "Wadsworth. 



In certain forms of instruments intended for angular meas- 

 urements only one vernier is provided, either because the 

 graduated arc is less than 360°, as in the case of the sextant, 

 position circle and other instruments of that class, or because, 

 as in certain forms of spectrometers, in which the vernier is 

 attached to the arm which carries the view telescope, it is of 

 advantage, from the point of view of mechanical simplicity or 

 convenience, to use but one. Except for the trouble involved 

 in determining and applying the necessary corrections to the 

 reading there is no serious objection to the use of a single ver- 

 nier, for if the mechanical work is reasonably well done little 

 change may be expected in the instrument even after very 

 considerable use. 



In this case however it becomes necessary to determine accu- 

 rately the errors of graduation — particularly those due to 

 eccentricity, and the method usually resorted to, is to remeas- 

 ure with the instrument whose circle it is desired to test, a 

 series of angles whose value has been first carefully determined 

 with another instrument whose constants are all known. This 

 method involves a very considerable amount of work even if 

 rendered possible by the possession of a suitable instrument. 



Recently the writer had occasion to test the error of eccen- 

 tricity of the graduated circle of a spectrometer, of the class 

 alluded to above, and as no other instrument was just then 

 available for checking its angular readings, the following 

 simple method was devised for this purpose. 



A piece of plane parallel glass (an ordinary sextant mirror) 

 is silvered on the face, a, fig. 1, and mounted on the prism 



stand, or on a bit of board provided with three leveling screws, 

 as shown, with a little soft wax or cement. The whole is 

 then placed on the spectrometer table and the mirror face set 



