Objectives for the Compound Microscope. 413 
micrometers, divided by the magnifying power, will give very 
nearly the same results as are obtained by the more complex 
rule, but this of course is not true for lower powers. 
It must, however, be constantly borne in mind that the re- 
sults obtained in any case are true only for the cover correction 
and distance used. 
For the convenience of those who may undertake such com- 
parisons, I append a table in which the real magnifying powers 
of single convex lenses are given for three different distances. 
In the treatises on optics the magnifying powers of single lenses 
are sometimes stated at some given distance from the lens to 
the screen, but I know of no table which shows their powers at 
given distances between image and object. 
This table is calculated by substituting the numerical values 
of fand Jin the equation f= mh when m= the magnify ; 
power remains as the only unknown quantity and is easily 
computed. I have carried out the values of m to two decimal 
— only; but in practice the nearest whole number will 
ound sufficiently accurate. 
Table of the magnifying powers of single convex lenses. 
Focal length for Magnifying power at 12}, 25, and 50 inches 
parallel rays, distance from micrometer to screen. 
~ 12 1-2 inches, 25 inches. inches. 
Sinvhes, © 2) EO 17 14°59 
Re ie eran ay 10°40 22°95 
1y “ By seis leat gay 14:59 81-30 
Isinohi)) ese wis wooeeo 22°95 47°99 
$rds of an inch, - - 16°69 35°47 42°98 
ithe: fica, 342 808 60°48 122°99 
f= - - 47°99 97°98 197°99 
$th ao 122-99 247°99 
Se ee 14799 297°99 
$th Ow Nac! gee 197°99 397°99 
oth el es SOROS 247-99 497°99 
th OY Ss oe eree 297-99 597°99 
sth oo a eee 37299 747-99 
ma . yy Tee 397-99 797°99 
ath... 4 -. +, . 999-00 447-99 897-99 
vth = 8% 5. 947-90 497°99 997-99 
sth ie Sn eee 622°99 1247°99 
wth =“ . . 622-99 1247-99 2497°99 
Note. —Since writing the foregoing article, I have read with 
Pleasure a paper on the same aehjent by Dr. R. H. Ward 
