ANNUAL ADDRESS OF THE PRESIDENT. LIIlI 
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 effect 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. 
Wan ui be oe 
3) oe . A o 
CWP Cle Cana E Dr 
Ties wns os 
Observer. = ES ire cas 
re alloy Cesk ooo 
wie | SS Sb sous 
3 2 Bu | ce BES 
3 3 ao aS) aoO8 
a = = cs > 
Inches. | Inches. 
1797 |; Sir Geo. Shuckburgh —.--__=__._ 14 Tat 1.50 | 0-01000 
tein peapt. Henry iater 28 oy ke 18 (aro See ee “00428 
SHSe ee CAM GIseis wily: =o sl ke la Le 27 (2.0) 3 00500 
Bee ree MUL yy. Wy eee ee (Oy fe “00500 
Boom) | We. mmcepsianks <5. es ae (2) | See ae 00358 
TeGenGens Avr. Clarkes, Heo. otuse 60 4. 0.67 00287 
1880 | Prof. W. A. Rogers, 1 in. obj. ---|__-__- LAs 8 ieee ae 00079 
ee ebb ODI. pee (Se.6) Po seoee ls 00035 
Ks a ae ON 2 PE eS (Bae ie ee 00019 
7.5 0.83 | 0:00394 
1888 | International Bureau ___________ 90 
Norr.—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 
116, p. 8. 
