ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
395 
tapped annulus of tlie frame of the exit slit. The instrument to be 
illuminated is brought close up to the diffusing tube, which is best 
distant about 1^ in. from the slit. 
The Limits of the Visible.* — Dr. A. Fock discusses the limits of 
the power of the Microscope, and in view of recent bacteriological dis- 
coveries, seeks to answer the question whether there may not be still 
many enemies to mankind, which our present Microscopes are unable 
to render visible. 
The magnification of a Microscope depends essentially on three 
factors, viz. the curvature of the lenses, the degree of perfection in their 
compensation of the so-called chromatic and spherical aberration, and 
lastly the angle of aperture of the objective. * 
Now the image formed by the Microscope will only perfectly corre- 
spond to the object if all the rays into which the incident light is split 
up by the dispersive effect of the object are again collected by the 
objective. If this condition is not fulfilled, an image, it is true, will 
result, but it will not perfectly correspond to the object ; and, according 
as different parts of the pencil are admitted into the Microscope, one 
and the same object can give quite different images. 
The angle enclosed by the rays resulting from diffraction on the object 
is smaller, the greater the number of diffracting parts. When those parts 
have dimensions many times greater than the wave-length the diffracted 
light forms a narrow pencil, and a small angle of aperture of the Micro- 
scope suffices in order to receive all the light. But when the dimensions 
of the parts of the object are comparable to or less than the wave-length, 
then the diffracted rays may occupy half the angular space. In this 
case the Microscope can no longer give a perfectly exact image, since 
the extreme anglje of aperture which the objective can give with the 
help of immersion lenses is 130°. 
Our best Microscopes can resolve with central illumination 2500 
divisions in 1 mm. Thus, with central illumination the minimum dis- 
tinguishable distance is O’ 0004 mm. or 0 * 4 /x. With oblique illumina- 
tion this can be reduced one-half, and by the use of the ultra-violet rays 
in producing photographic pictures to 0*12/4. All objects and struc- 
tures then, which do not reach these limits, must for ever remain hidden 
from the human eye. 
Now determinations of the size of molecules, made from the kinetic 
theory of gases, have given for water the molecular diameter of 0 • 00017 /x, 
and for carbonic acid 0 * 00009 /x. The smallest visible object is thus 
about a thousand times larger than a molecule. From the stand-point 
of the atomic theory therefore, the existence of an invisible world of 
life, which would represent a continuation of the micro-organisms known 
at the present day, is not altogether impossible. 
C6) Miscellaneous. 
The late Prof. Fol.j — M. M. Bedot has a biographical notice of 
our late Honorary Fellow, whose fate will, we suppose, remain for ever 
wrapt in mystery. Hermann Fol was born near Paris, on July the 23rd, 
1845 ; at Geneva, whence his parents came, he was brought under the 
* Central-Ztg. f. Optik u. Mech., xv. (1894) pp. 76-8. 
f Arch. Sci. Phys., xxxi. (1894) pp. 264-83. 
