ANNUAL ADDRESS OF THE PRESIDENT. 
LTII 
and, second, because they avoid the injury to standard scales which 
necessarily results from placing the points of beam compasses upon 
their graduations. As the objective of the microscope forms a mag¬ 
nified 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 efiect of imperfections in the micrometer, the objective should 
have the largest practicable magnifying power. To show the pro¬ 
gress 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 accompanying table. 
Observer. 
^ s S 
Inches. 
Inches. 
1797 
1817 
1834 
1884 
1850 
1864 
1880 
1883 
Rir Goo. Shnokburgb . . . 
14 
18 
27 
Capt. Henry Hater _ __ _ _ 
Francis Baily „ _ _ _ 
Lieut. Murphy. R. E. 
R. Sheepshanks _ _ 
Gen. A. R. Clarke, R. E_ 
60 
Prof. W. A. Rogers, 1 in. obj. __ 
“ “ J in. obj. 
“ “ in. obj. . _ 
International Bureau _ __ 
90 
1.7 
( 2 . 3 ) 
(2.0) 
1.50 
f 
(2.0) 
(2.8) 
4. 
0.67 
(12.7) 
(28.6) 
(52.7) 
7.5 
0.83 
0-01000 
•00428 
•00500 
•00500 
•00358 
•00287 
•00079 
•00035 
•00019 
0 00394 
Note.— The magnifying power of Sir Geo. Shuckburgh’s microscope 
seems to be referred to a distance of twelve inches for distinct vision. The 
powers inclosed in parentheses 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 respect¬ 
ing the method adopted by the Academicians for comparing their 
various toises; but in his astronomy, Lalande states that the com¬ 
parisons were effected partly by beam compasses, and partly by 
superposing the toises upon each other and examining their ends, 
both by touch and with magnifying glasses ; they being all end 
standards.^ For the definitive adjustment of the length of their 
* 
I 
16, p. 8. 
