ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
681 
The cover-plate, to which the camera-bag is glued, has a depression 
in its upper surface, 18 X 18 cm. and 2 mm. deep, in which fits (not 
slides) the slide-holder. The slide-holder consists of a wooden plate 
with a thin brass plate on its upper surface. The wooden plate has a 
central round aperture of 7 cm., the brass plate on which rests the 
photographic plate, one of 6 cm. In the wooden plate is a shutter 
15 cm. long and 8 cm. broad. The photographic plate is held in a 
frame on the slide-holder, and is protected above by a light-proof cover 
carrying a spring which presses it down upon the metal plate of the 
holder. 
The author describes in detail the process of taking a photograph 
with the apparatus. As source of light he prefers the Auer incandescent 
gas-flame. 
( 5 ) Microscopical Optics and Manipulation. 
Tests for Microscope Objectives.* — Mr. E. M. Nelson enumerates 
for “ Beginner ” some useful test-objects for microscopic objectives. 
With a 3/4 axial cone a “ spread slide” of P. angulatum , dry on cover, 
is a good test for lenses from 1/2 in. upwards. One of the best diatoms 
for higher powers is the large N. rhomboides found in “ Sozodont” tooth- 
powder. The best test for low-power lenses is a balsam-mounted 
diatom with dark-ground illumination by Abbe condenser and central 
stop. 
“ The golden rule for the resolving power of any objective with a 3/4 
axial cone of illumination, is that they should show a fineness of 
structure equal to 70,000 multiplied by their N. A.” 
On the Theory of Optical Images with special reference to the 
Microscope. f — Lord Bayleigh remarks that the theory of optical images 
has been treated from two distinct points of view. The method adopted 
by Helmholtz consists in tracing the image representative of a mathe- 
matical point in the object, the point being regarded as self-luminous ; 
while in the method of Abbe the typical object is not a luminous point , 
but a grating illuminated by plane waves. In the latter method, it is 
argued that the complete representation of the object requires the co- 
operation of all the spectra which are focused in the principal focal 
plane of the objective ; when only a few are present, the representation 
is imperfect, and wholly fails when there is only one. From these con- 
siderations the resolving power can be deduced as follows : — In fig. 103, 
A B represents the axis, A being in the plane of the object (grating) 
and B in the plane of the image. S 0 S 15 &c. represent the diffraction 
spectra. From the central one S 0 the rays diverge and illuminate a 
circle C D in the plane of the image. The first lateral spectrum S x is 
due to rays diffracted from the grating at a certain angle ; and in the 
critical case the region of the image illuminated by rays from S x just 
includes B. Since the extreme ray S x B proceeds from A, the condition 
that S : shall co-operate at B is that the angle of diffraction do not 
exceed the semi-angular aperture a. But the sine of the angle of 
diffraction is X / e, where X denotes the wave-length and e the smallest 
resolvable distance. The above condition then requires that e exceed 
X / sin a. With oblique incidence the limit becomes ^ X/ sin a. 
* English Mechanic, lxiv. (1896) p. 187. 
t Phil. Mag., xlii. (1896) pp. 167-95 (4 figs.), 
