449 



XV. — A Report on the Grayson! s Rulings presented by Mr. C. Beck 

 to the Royal Microscopical Society. 



By E. J. Spitta. 



{Read April 19, 1911.) 



Some months ago the Council requested me to examine and report 

 upon two sets of rulings by Grayson — an inch divided into hun- 

 dredths and a millimetre with divisions of ■ 25 — gifts to the Society 

 by Mr. Conrad Beck. 



The examination presented many difficulties not anticipated at 

 the commencement, one being the selection of a suitable arrange- 

 ment that could not possibly shift or change in any way whilst in 

 use from day to day during the necessarily protracted observations. 

 Several hundreds of experimental measures extending over a some- 

 what lengthy period were made before the arrangement finally 

 adopted was perfected and accepted as satisfactory in all respects. 

 Then, again, owing to the fact that no standard inch or millimetre 

 scales were in existence, no comparisons with such standards could 

 be made, and only observations of a differential nature therefore 

 could be carried out. To make these measures as effective as pos- 

 sible, before settling down to obtain the final results, several 

 preliminary experiments were tried with different objectives and 

 oculars to ascertain what optical combination lent itself as most 

 suitable to the subject in hand ; and many suggestions and ideas 

 were discussed with Mr. Conrady, who very kindly associated him- 

 self with the work, and who is responsible for the reductions of all 

 the measures, so that the best possible information could be obtained 

 therefrom. 



It may be well to point out here that, owing to the actual size 

 of the spaces, it was impossible to span (at least with a satisfactory 

 definition at the edges of the field) more than two intervals of the 

 scales on either side of the fixed thread of the screw micrometer 

 used, hence in a mathematical point of view only approximate 

 determinations of the progressive errors (about to be mentioned) 

 in the scale could be hoped for. 



It" may now be at once mentioned that the probable error of 

 one single setting was found to be + • 124 fi 2fioVoq * n - m tne 

 millimetre scale, and + • 2 /* X2 500J5. in - in tne case ol tiie iuch > 

 but as in the final observations each interval was really measured 

 six times, the results were correspondingly more accurate, having 

 errors of ± 0-05 ^ s^oWo in - and ± °' 08 P 3T2 l 5oo. in - respect- 

 ively, which compare very favourably with other published obser- 

 vations on scales of this type. 



