608 



MISCELLANEOUS PAPERS. 



the eight huudredtli part of au inch directly, and the tenth of that 

 luautity by estimation; but Mr. Graham did not consider that the 

 measurements could be depended upon to a greater accuracy than one 

 sixteen-hundredth of au inch.* 



Troughton is generally regarded as the author of the application of 

 micrometer microscopes to the comparison of standards of length, but 

 the earliest record of their use for that purpose is by Sir George 

 Shuckburgh in his work for the improvement of the standards ot 

 weight and measure, in 179G-'98.t Since then their use has been gen- 

 eral; first, because they are more accurate than beam compasses, and 

 second, because they avoid the iujury to standard scales which neces- 

 sarily results from placing the points of beam compasses upon their 

 graduations. As the objective of the microscope forms a magnified 

 image of the standard, upon which the micrometer wires are set by the 

 aid of the eye-piece, it is evident that in order to reduce the effect of 

 imperfections in the micrometer, the objective should have the largest 

 practicable magnifying power. To show the progress in that direction 

 the optical constants of the microscopes, by means of which some of the 

 most important standards have been compared, are given in the accom- 

 panying table : 



Date. 



Observer. 



Power 

 of micro 



Mag- j Equlva- 

 iiifviuj; lent focus 

 , power of of eye- 

 scope, oi.jeciive. piece. 



Value of 

 one revo- 

 lution of 

 luicronie- 

 tw screw. 



1797 

 1817 

 1834 

 1834 



1850 

 1864 



1880 



1883 



Sir George Shuckbnrgli 



Captain Henry Kater 



Francis Baily 



Lieutenant Murphy, R. E 



R. Sheepshanks 



General A. R. Clarke, R. E 



Prof. W. A. Rogers, 1 inch objective 

 Prof. W. A. Rogers, ^inch objective 

 Piof. W. A. Ro<i;ers, | inch objective 

 International Bureau 



14 



18 



27 



60 



90 



1.7 



(2.0) 



(2. 0) 



(2. 8) 



4. 



(12.7) 



(28. 6) 



(52. 7) 



7.5 



Inches. 

 1.50 



0.67 



0.83 



Inches. 



0. 01000 

 . 00428 

 . 005IJ0 

 . 00500 

 . 00358 

 . 00287 

 . 00079 

 . 00035 

 . 00019 



0. 00394 



Note. — The magnifying power of Sir George Shuckburgh's microscope seems to be 

 referred to a distance of 12 inches for distinct vision. The powers inclosed in i)aren- 

 theses are estimated upon. the assumption that the respective micrometer screws had 

 one hundred threads per inch. 



In the memoirs of the French Academy, nothing is said respecting the 

 method adopted by the Academicians for comparing their various toises ; 

 but in his astronomy, Lalande states that the comparisons were effected 

 partly by beam compasses, and partly by superposing the toises upon 

 each other and examining their ends, both by touch and with magnify- 

 ing-glasses; they being all end standards.^ For the definitive adjust- 

 ment of the length of their meters, which were also end standards, the 

 French Metric Commission used a lever comparator by Lenoir. 



8, pp. 545-'r). 



121, p. 137. 



1 19, p. 8. 



