Monthly Microscopical 
Journal, Sept. 1, 1870. 
154 On the Focal Length 
most probable mean of a number of observations. The column 
headed B gives the focal length indicated by the maker in decimals 
of aninch. The next column, headed “ diff. a,” gives the differ- 
ence between the actual focal length, as determined by these obser- 
vations, and the designated focal length as given in column B. The 
last column, headed “ diff. B,’ gives the difference in decimals of an 
inch between the eatreme value of f as given from different obser- 
vations by this method. 
The micrometer used as an object was a glass ruled scale divided 
to +45 mm. (355 inch), except in the objectives numbered 9, 10, 
16, 18, 28, 31; 32 (marked with an asterisk), in which a y;'59 inch 
stage micrometer was substituted, as the more finely divided scale 
could not be read with such low powers as these objectives gave. 
Objectives Nos. 16 and 28 are single lenses; the rest are triplets. 
Lens No. 32, by Zentmayer, had been slightly altered in focal 
length to adapt it to a gas microscope. The adjustment for covered 
objects shortens the focus of all the glasses examined, except Nos. 11, 
12, and 17, in which a lengthening of focus takes place. No. 11 
is adjusted for glass covering when the index is at o, contrary to 
the usual method in which the greater the number of the index the 
thicker the covering for which the objective is adjusted. Objective 
No. 12 is No. 11 with the front lens removed and an “ immersion 
front,” a lens of different focus screwed on in its place. 
No. | Name of Maker. A Ciae ae Focal Length. | B. | Diff. A. Diff. B. 
| mm. | mm. in. in, in. in. 
la! Hartnack, No.10. | Une. Dry | 249 | 1°985 | -0782 0006 
b| note sored Cov’d Dry | 249 | 1:696 | -0668 “0009 
c | | Une. Wet | 249 | 1-991 | -0784 “0002 
d al casei | Coy'd Wet | 249 | 1°681 | -0662 “0002 
2 | Hartnack, No.9 .. | Unc. Dry | 249  2°317 | -0912 *0007 
| | Une. Dry | 264 | 2-325 | -0915 *0012 
| Cov’d Dry | 248 | 2:079 | -0818 “0003 
Cov'd Dry | 249 | 2:073 | -0816 | “0001 
Coy’d Dry | 261 | 2-079 | -0617 | “0006 
Une. Wet | 261 | 2:339 | -0921 |} .. ee “0007 
3 | Hartnack, 3 in. | ds | 249 | 6-295 | -2478 | +2500 |—-0022 |-0013 
4 | Hartnack, din. .. | e | 259 13°659 +5377 | -5000 |4+ 0377 |-0027 
5a! Nachet, No. 7 . | Une’d | 265 | 2°279 | -0897 AP ar “0002 
b ror re Ao Cov'd 265 | 1°827 | -0719 “0003 
6 | Nachet, No. 5 3 264 | 3°066 | -1207 “0004 
7 | Nachet, No. 3 5 235 | 4°666 | -1758 “0004 
8 | Nachet, No. 2 ne | 239 | 6:469 | +2547 “0006 
9* | Nachet, No. 1 os | 244 14-791 | +5823 “0012 
10* Nachet, No. 0 Ns 268 39°103 1°5395 yr *0027 
11 a| Ross’s +, in, .. 0 249 | 1-675 | -0659 | .. 5 0004 
b Oe Ege ee 22 250 1-900 | -0748  *0833 |— +0085 |-0013 
Ross’s +4; in., with : 
12a { immersion front) 0 Dry 249 1-265 -0498 0001 
by Tolles .. 
b he ee 0 Wet 249 1:273 +0501 | (0006 
