Objectives for the Compound Microscope. 73 
obtained in any case are true only for the cover correction and dis- 
tance 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 some- 
times 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 
11% l 
f and l in the equation / = > , when m = the magnifying 
power remains as the only unknown quantity, and is easily computed. 
I have carried out the values of m to two decimal places only ; but 
in practice the nearest whole number will be found sufficiently 
accurate. 
Table of the Magnifying Powers of Single Convex Lenses. 
Focal length for parallel rays. 
Magnifying power at 12£, 25, and 50 inches 
distance from micrometer to screen. 
3 inches 
12^ inches. 
1-50 
25 inches. 
6-17 
50 inches. 
14-59 
2 
4-00 
10-40 
22-95 
li inch 
6-17 
14-59 
31-30 
1 
% rds of an inch . . 
10-40 
22-95 
47-99 
16-69 
35-47 
72-98 
T^ths „ .. .. 
29-21 
60-48 
122-99 
ith ., .. .. 
47-99 
97 -9S 
197-99 
ith „ .... 
60-48 
122-99 
247-99 
Lth „ .... 
72-98 
147-99 
297-99 
I th 
97-98 
197-99 
397-99 
*th „ .. .. 
122-99 
247-99 
497-99 
Tfcth „ .. .. 
147-99 
297-99 
597-99 
iVth „ •• •• 
185-49 
372-99 
747-99 
iV th » •• •• 
197-99 
397-99 
797-99 
iV th » 
222-99 
447-99 
897-99 
■Jjjth „ . . . . 
247-99 
497-99 
997-99 
T5 th >• •• •• 
310-49 
622-99 
1247-99 
Toth „ .. .. 
622-99 
1247-99 
2497-99 
Note . — Since writing the foregoing article, I have read with 
pleasure a paper on the same subject by Dr. R. H. Ward.* This 
paper contains much valuable matter, and should be read by all 
who are interested in this subject. I learn from it for the first 
time that the method of determining the magnifying power of 
objectives by removing the field-glass of the eye-piece and using 
eye-piece and stage micrometer as described above, has already been 
* “ Remarks on Uniformity of Nomenclature in regard to Microscopical 
Objectives and Oculars,” ‘American Naturalist/ March, 1872, p. 136. 
