690 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
with the old method which consisted in giving the numbers of the 
eye-piece and objective. In order to judge of the value of a drawing, 
the indication of the magnification in diameters is not sufficient, at least 
in the case of high magnifications : in these cases the particular eye-piece 
and objective used should also be given. In illustration of his conten- 
tion the author gives two examples in which magnifications of 250 and 
370 times can be obtained by four different combinations, in each case, 
of objectives and eye-pieces of Zeiss. 
Theory and Technique of the Microscope.* — This contribution 
from the literary remains of Dr. Th. Marsson consists of notes on the 
theory and technique of the Microscope, arranged under the following 
headings : — General optical principles ; Dioptrics ; Lenses ; Objective 
systems ; Focal length ; Angle of aperture of the system ; Eye-piece ; 
Magnification ; Field of view ; Aberration. 
The information conveyed is for the most part such as is given in 
more extended form in the ordinary text-books, but may be found useful 
as a review of the more important facts to be remembered in Micro- 
scopical work. A few points may be noticed. Under the heading of 
Objective-systems the influence of the cover-glass is discussed. Diver- 
gent rays from the object passing through the cover-glass produce a 
series of images one above the other, the effect of which is similar to 
that of spherical aberration. With weak objectives the effect is negli- 
gible, but increases with the strength of the objective and with the 
thickness of the cover-glass. The thickness of the cover-glass also 
stands in a certain relation to the length of the body-tube : the more the 
tube is shortened, the thicker must be the cover-glass. 
The effect of immersion systems is to give correction of aberration, 
increase of magnification and of angular aperture, increase of the distance 
between objective and object, and admissibility of thicker cover-glasses. 
In the use of the correction arrangement, the Microscope is adjusted on 
the dark edge of an object, and the lenses are brought nearer together or 
farther apart, until an equally strong broadening of the edge results, 
whether the object be brought within or beyond the principal focus. 
The objective system should be somewhat over-corrected, the eye- 
piece a little under-corrected. 
The methods of Gray and of Govi for determining the angular 
aperture are described. In the Govi method the Microscope is in the 
ordinary vertical position on a dark table. A lens of from 2 to 3 cm. 
focal length is brought above the eye-piece, and two strips of white 
paper are placed on the table near the Microscope. These are separated 
from each other until their images just come on the opposite edges of 
the field of view. If the distance between the two strips of paper = d, 
the distance of the objective from the table = a, the tangent of the half 
angular aperture is 
For testing spherical aberration, a mercury thread in a capillary 
tube is recommended for weaker objectives, and fine scratches on silvered 
glass for stronger objectives. 
* Zeitschr. f. Angewandte Mikruskopie, i. (1895) pp. 33-7 and 65-9. 
