ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
257 
resolution), then for their proper work they are better lenses. My later 
purchases, for students’ ordinary use, have been of 110° air angle for 1/5 
in., with the expectation that anything up to the widest numerical 
aperture may sometimes be accessible. For the closest possible studies 
upon the exact size (measurement) and shape of small stained bacteria, 
a Tolies’ 1/15 in. homogeneous immersion of 123° balsam angle is 
the best I have used, though others at hand have considerably wider 
aperture. 
Get the best. — Having decided what is most suitable for the work 
proposed, the very best should be selected for students’ use, as well as 
for special investigators. It may be said that the expense would often 
be too great, and that cheap instruments or none constitute the alter- 
native. Often, however, this is the mere outgrowth of too cheap ideas, 
either on the part of the instructors or boards of trustees. If the real 
needs are fairly appreciated, in this as in any other case, they can usually 
be met in some way. Otherwise, how are Microscopes obtained at all ? 
At any rate, instructors should inform themselves with the utmost care 
and then equip their pupils in the best possible manner with this, the 
most delicate of all tools. No questions of home or foreign manufacture, 
of accidents of popular approval or of hereditary service, should be 
allowed weight in the selection of a Microscope objective. Neither 
should the cost price be taken as an index of quality. No one can be 
blamed for buying what he finds to be the best goods for the least 
money. 
Governed by these principles, T have ceased ordering from abroad, 
for students’ use. Without naming other makers, I choose the ob- 
jectives of the Bausch and Lomb Optical Company, in preference to those 
of Leitz. I have in daily use some first-class wide-angle dry objectives 
of the Gundlach Optical Company, that have given most excellent 
satisfaction. Anxious to have the best, as improvements were announced, 
I have ordered, from time to time, five first-class objectives, each one 
supposed at the time to be the very best in the market. This paper 
may seem less presumptuous with this statement inserted. 
Specific Tests. — I am now to report the results of some comparative 
tests, made with certain named objectives, under described methods of 
procedure. When the title of this paper was announced I hoped to have 
photographs taken in different ways for each objective tried, but have 
found too much time consumed in other directions to permit it. 
Please allow me to express the conviction, that these proposed photo- 
graphs would have certainly corroborated the statements herein made. 
In order to decide, with certain correctness, of the relative quality 
of the objectives compared, the tests were purposely made as difficult 
as circumstances permitted, but under these difficulties each was given 
the best handling possible for the manipulator. The objectives, all 
homogeneous immersions, were as follows: — Tolies’ 1/15 [1880] 123°; 
Zeiss’ apochromatic 1/12 [1887] N.A. 1 • 40 ; Herbert Spencer’s 1/10 
[1888] balsam angle 130°; Gundlach’s 1/12 [1890] balsam angle 136°. 
The last was asked for and loaned to me for trial. An attempt was 
also made to include a Leitz 1/12 [1888] N.A. 1-25, but it was not 
possible to use it on the same stand, hence not certainly under the same 
conditions, and not included in this report. I have not been able, 
