ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
263 
graphs which were prepared about the year 1860 by a physician, Dr. 
Henderson, residing in Mobile, Alabama. The collection consists of 
sixty-five silver prints, each 6 in. in diameter, mounted on a page of an 
album about 12 in. square. Traced on the negative with a pin are tlio 
words “Dr. Herapath’s 1/4 in. lens, ‘Ross,’ April 25th, 1860.” The 
name of the object and the magnification employed are given with each 
photomicrograph. The collection includes specimens derived from 
insect or parasitic life, specimens of diatoms, specimens from plant life, 
specimens from sea life, and histological specimens. 
Photographing Plate Cultivations.* — In the usual method of light- 
ing for photographing plate cultures, says Mr. G. F. Atkinson, the finer 
characteristics are often lost, and in case of very transparent colourless 
organisms the image is throughout very dim. By covering the bottom 
and top of the culture dishes (Petri’s capsules) with an opaque screen, 
the light is admitted only through the sides of the vessel, and the object 
is thus photographed by reflected instead of transmitted light. In this 
way very clear and minute details are obtained. 
(5) Microscopical Optics and Manipulation. 
Theoretical Limit to the Capacity of the Microscope.! — Herr 
K. Strehl gives an investigation of the limit to the resolving power of 
the Microscope. In his book ‘Theorie des Fernrohrs auf Grund der 
Beugung,’ he has given a theoretical proof on the ground of diffraction 
that the limit of the resolution of a double star occurs for the value 
Z = 3-2. 
Here Z = — — , where r denotes the radius of the aperture of the 
wave-surface, p the radius of the wave surface or the focal length, <r the 
distance apart of the two points of light in the focal plane, and A the wave- 
length of the light, all measured in mm. 
Taking the case of a pencil of rays from the Microscope objective, 
and considering wave-surfaces drawn as normals to the rays through the 
hinder focus, p will represent the so-called “ optical tube-length,” and r 
must be measured in the hinder focal plane of the objective. Now 
according to Abbe r — f a where / is the hinder focal length of the 
objective and a its numerical aperture. If e denote the minimum dis- 
tinguishable distance in the object, then in the plane of the image 
V 
°' = 7 ' 
We have now, corresponding to the limiting value Z 
following : 
3*2 Xp 1 • 6 Aw 1 • 6 A 
<T = -5 ; V = jr- ; € = 
A tt r 7va f 7 t a 
3-2, the 
0-5093 -■ 
a 
Thus for extremely obliquely illuminated objects e = 0 • 5 - ; and for 
( % 
A = 0*00055 mm. and a = 1 we obtain e = 0*28 >, i. e. 3570 striae in 
* Bot. Gazette, xviii. (1893) p. 333. 
t Central-Ztg. f. Optik u. Mechanik, xiv. (1893) p. 277. 
