ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
689 
correction for cover-glass thickness is particularly necessary in the case 
of the apochromatics 4*0 and 3*0 mm. (dry) and 2*5 mm. (water- 
immersion). 
(4) Photomicrography. 
Zeiss’ Photomicrographic Stand.* — The stand shown in fig. Ill is 
in size and general arrangement similar to the Zeiss stand I a. The 
round rotating stage is of brass, and has a diameter of 100 mm. By 
means of the co-axial milled heads H and Y, it can be moved in two 
rectangular directions and its position can be read off on the verniers. 
The body-tube is very short and unusually wide, so as to provide for the 
use of long focus lenses. 
(5) Microscopical Optics and Manipulation. 
Diffraction Theory.^ — Herr K. Stralil remarks that the knowledge 
of the diffraction theory is daily becoming more important for the prac- 
tical optician. It becomes more than ever necessary therefore to 
understand what is meant by diffraction-theory. According to the 
author there are two diffraction theories which are constantly being 
confounded. One diffraction theory relates to the modification which 
light suffers in passing across sharp edges or between narrow slits. 
The diffraction, however, which is of such importance for the telescope, 
for observation and measurement, is something quite different from this. 
Here the edges and diaphragms act not as edges consisting of brass or 
steel, but as geometrical boundaries which prevent the incident light- 
wave being effective in its whole extent. The greater the effective part, 
the smaller is the effect of the diffraction. 
Compensation of Errors of Objectives, t — Herr K. Strahl gives some 
of the results which follow from the calculations of the most important 
errors of objectives which he has made. 
According to geometrical optics, it is easy to conceive the possibility 
of correcting the positive spherical aberration of an objective by the 
negative equally great one of an eye-piece. 
According to geometrical optics, it is, however, impossible to correct 
the positive error of an objective by an equally great positive error of 
the eye-piece. 
And yet this is, according to the diffraction theory, perfectly possible. 
The author, by a closer investigation, finds that the light-distribution 
in the region of the focus of an objective of which the spherical aberra- 
tion A is < 1 (for yellow rays this is the case with all newer objectives) 
is so similar to that of an aplanatic objective, that the difference amounts 
to scarcely more than one-hundredth of the complete light-intensity. A 
compensation of such errors is therefore unnecessary. 
The author accordingly concludes that the influence of the spherical 
aberration has been up till now considerably overrated by the theorists. 
Indication of Magnification in Micrographic Drawings.§— M. H. 
Bolsius, referring to Dr. Carazzi’s note on this subject, || states that he 
considers that the method advocated in that note should be combined 
* Zeiss’ Catalogue, No. 30, 1895, pp. 40-1. 
f Central-Ztg. f. Optik u. Mechanik, xvi. (1895) p. 213. 
t Tom. cit., pp. 183 and 194. § Zool. Anzeig., xviii. (1895) pp. 386-8. 
|| See ante , p. 368. 
1895 2 z 
