ZOOLOGY AND BOTANY, MICROSCOPY, ETO. 
677 
known, it fulfils the conditions of achromatism, viz. that A = 
P +P 
where the letters have their usual meanings. It is clear that P and p 
may have an infinite number' of separate values, and may yet give the same 
value for A. Which of these infinite values are the best? For the last 
century it has been usual to put P = Sp, and “ Huyghens’ ocular ” usually 
has this construction. Dr. Schroeder admits that in this form the com- 
bination is not ill adapted for high magnifications, but considers it ill 
suited for w 7 eak magnifications, and the nearer A approaches the value 
unity the worse the combination is. The necessary insertion of a stop 
which excludes half the field of view is a grave defect. He has there- 
fore recalculated the equations, and after several years’ experience gives 
a table for the decimal values of P from 1 to 3, from which the following 
lines are extracted. 
The conditions underlying the table presuppose (T) that both lenses 
are plano-convex ; (2) that they are of the same material ; (3) that in the 
first approximation the various coloured images are brought to the same 
size ; (4) that the eye-point is exactly at the half-focus of the eye-glass, 
in order that the observer should not come too near the lens. 
F = Objective focus. 
P = Field-lens focus. 
p = Focus of the eye-lens. 
A = Distance of both lenses with respect to their cardinal points. 
s = Distance of field-lens from the objective. 
E = Equivalent focus of the ocular. 
L = Length of the instrument from the first to the last lens. 
m = Magnification of the instrument. 
d = Distance of diaphragm from the field-lens. 
D = Distance of objective image from the field-lens'. 
/ = Focal length of the primary rays after quitting the field-lens 
measured therefrom. 
8 = Diameter of the diaphragm in the focus of p. 
® = Half the visible field of view. 
p 
\v 
A 
s 
F 
m 
d 
D 
E 
/ 
L 
2 0 
5 max. 
1-0 
1 
1-00 
2-00 
2 
2 
0 
0 
! l 
2-00 
3-00 
o 
28 
58 
0-500 
1*5 
1 
11*25 
4*50 
4-80 
4*00 
0-25 
0-300 
1-200 
2-25 
5*75 
38 
56 
0-667 
2-0 
1 
1-50 
10-00 
10-67 
00 
© 
© 
0-50 
0-667 
1-333 
2-50 
11*50 
47 
10 
0-800 
2*5 
1 
1*75 
27*50 
28-57 
20-00 
0-75 
rH 
r-- 
© 
1-429 
2-75 
29-25 
54 
4 
0-909 
3*0 
1 
2*00 
1 
00 
00 
00 
1-00 
1-500 
1-500 
3-00 
60 
0 
1-000 
New Microscope Objective for Zoological and other Biological 
Investigations under Water.* — Dr. H. Hartwig, of Jena, describes the 
above apparatus, which has been made to his designs by the Zeiss firm. 
* Zeitscbr. f. wi?s. Mikr. xv. (189S) p. 102. 
