ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
Ill 
The microscopist is therefore not obtaining such results as his objectives 
should enable him to obtain, and the efforts of the conscientious optician 
to provide classified objectives of reliability and similar performance are 
almost entirely nullified. In making the necessary experiments some 
astonishing results appear. With a non-adjustable dry 1/5 corrected 
for a cover-glass of O' 16 mm., employing the extremes of cover-glass 
Fig. 7. 
THOUSANDTH! , lo 
, Of INCH | 1 \CL 
|3 |4 
I s 
[6 
11 
18 19 
ho 
III |I2 
|I3 , 
|I4 1 15 |I6 ( 17 ||8 1 19 
HUNDREDTH OFMtti 1 
1 1 1 1 1 1 1 
1 1 '1° 1 1 
1 1 
nn 
I 1 2 I° 1 l l l 
1 1 1 
l l 3|0 | | 
f 1 1 
1 1 I I 40 1 1 1 ill I I 1 5(0 
NON adjustable. f J[ 
li '4 
94 
24 
H 
1% 
6% 
6 
S4 
D A 1 1 O ^1 1 O 1 A Alin AnT 1 A a 1 A n 
OBJECTIVES | 
CORRECTED FOR rS 
13 
10 
84 
8 
7 
64 
5/a 
4% 
— BAUSCrl & LOMB OPTICAL C 9 
■ - Rnrur ctfq MV Mc\a/ Yaru Pitv 
046Mm COVER CLASS) 
15 
II 
84 
IX 
6 
4/i 
nULMt-5 It-N .|N.r. o< INt-W iUr\l\vllT. 
lUBEUNClM^INtMES^ - 
\Vi 
\2>i 
84 
6 
4 
TUBELENOTH ISO Mnv& 
240 Mm 
190 
160 
135 
110 > 
which are used by the various manufacturers as standard as obtained by 
efforts of Prof. Gage, I found that for 0*25 mm. a tube-length of 6 in. 
is required to obtain the proper correction, while for a thickness of 
0*1 mm. 13 in. of the tube-length are necessary. In a 1/8 objective 
adjusted under the same conditions, 4J in. are the requisite for a cover 
of 0*25 mm., and for 0*1 mm. 15 in. The further fact is shown that 
with a 1/5, which under conditions of tube-length and cover-glass given 
above shows certain structure well defined, absolutely fails to show 
anything of it under a cover-glass of 0*1 mm. on one side and 
0*25 mm. on the other, and further a marked chromatic over or under 
correction. With a cover of 0*14, which would seem but a slight 
variation from the standard, the objective is spherically highly under- 
corrected, and with 0*18 highly over-corrected. With objectives of 
high power the difference is still more marked. For these experiments 
I have had Mr. J. D. Moller, of Germany, mount a series of Pleurosigma 
angulatum dry and Amphipleura pellucida in balsam under a series of 
covers varying from 0*1 mm. to 034 mm. each carefully measured and 
marked. I have used these objects because they are my favourite tests, 
and it goes without argument in saying that any preparation showing 
structure under the above objectives will be affected to the same extent 
by the varying conditions of cover-glass as these objects, and in objects 
of still finer structure the limit of visibility will be reached corre- 
spondingly sooner. 
The system which I have devised to aid in overcoming these diffi- 
culties depends in the first instance upon a micrometer for measuring the 
thickness of cover-glass. While the delicate instruments made by M. 
Grossman, of Germany, are excellently suited for this purpose, they are 
expensive. I have endeavoured to overcome this objection by construct- 
ing a plain screw which, while not so sensitive to the touch, is suf- 
ficiently so for all practical purposes. The instrument is provided with 
a stand of japanned iron. Out horizontally through the top is a thread 
of 1/50 in. pitch, and 3/16 in. outside diameter ; a recess is cut on the 
top below the line of the screw, and at right angles to it for placing the 
covers. The one-half of the top of the stand which receives the micro- 
meter screw is slotted longitudinally to the depth of the screw, and is 
provided with a set screw to take up wear. The other half has the 
