NALURE | 
THURSDAY, MARCH 
28, 1907. 
ULTRAMICROSCOPES. 
Les  Ultramicroscopes. Les  Objets — ultramicro- 
scopiques. By MM. A. Cotton and H. Mouton. 
Pp. 232. (Paris: Masson et Cie., 1906.) 
HE magnitude of an object which can be rendered 
visible by the ordinary use of the microscope 
1 which is well understood and can 
be succinctly expressed. Jt depends not merely upon 
the construction of the instrument, but also upon 
the character of the light employed and upon the 
liquid used for immersion. The instrument should 
possess a large numerical aperture, which is again 
increased by immersion in the ratio represented by 
the index of the immersing liquid, the result being 
the scientific expression for the power of the instru- 
ment, with a given magnifying power, to resolve 
close lines or points. As regards the light itself, the 
limit of resolution is proportional to the wave-length, 
so that shorter wave-length implies greater power of 
resolution. When special light is selected for 
employment, the mean value of the wave-length is 
0-55 #, where “ signifies 0.oo1 of a millimetre. 
Taking full advantage of these principles and of 
the high index, 1-66, of monobromonaphthalin as an 
immersion liquid, it may be said that the smallest 
visible objects have a magnitude not less than 0.17 x. 
Bodies smaller than this are called ultramicroscopic. 
Some plan other than the usual microscope method 
must be adopted in order to make their existence 
appreciable, and it is upon this subject that MM. 
Cotton and Mouton have written the very valuable 
and learned book before us. In it will be found 
accounts, not merely of their own work, which is far- 
reaching and in practical points highly ingenious, 
but also of that of other investigators in the same 
field. 
There are two methods at present 
which may be called respectively that of ultra-violet 
light and that of diffraction in a dark field. The 
first method aims at taking advantage of the short 
length of ultra-violet wave-lengths. The sources of 
light are electric sparks formed between wires, which 
may be of magnesium, producing wave-lengths of 
0.280 #, or of cadmium, producing those of 0.275 4, 
the former being more intense, the latter more homo- 
geneous. Such waves produce no effect upon the eye, 
though much upon fluorescent screens and photo- 
graphic plates. But they are readily absorbed by 
glass. Hence the media (excepting air and immers- 
ing liquids) through which they pass on their way to 
the fluorescent sereen or photographic plate, as the 
case may be, must be of quartz, and those above the 
stage of the microscope must, to avoid effects of 
double refraction, be of fused quartz. Thus the whole 
apparatus is highly specialised. On the other hand, 
the rays employed being, homogeneous, there is no 
chromatic aberration to be considered in the design 
of the lenses. 
NO. 1952, VOL. 75 | 
has a lower limit 
not 
in existence, 
The image formed by the objective is again magni- 
fied by the ocular, employed in such a 
form a second real image at the place where finally 
way as to 
is placed the fluorescent screen or photographic plate. 
With such an apparatus the limit of magnitude of the 
objects detected would be reduced to 0-09 xz. 
The second and more recent method of detecting 
ultramicroscopic bodies is to employ their power of 
diffracting the light which falls upon them. They 
thus become mere point sources of light, but. diffrac- 
tion discs are formed upon the retina of the eye, as 
in the case of stars the dimensions of which are far 
too small to subtend an appreciable angle, even with 
the most powerful telescopic aid. 
In the microscope, then, the illuminated ultra- 
microscopic object merely appears as a star of light. 
The form of the object is entirely unobserved, its 
presence only being appreciable when certain con- 
ditions are fulfilled. These are that the illumination 
shall be intense, that the field shall be profoundly 
dark, and that the objects themselves shall be 
sufficiently sparsely distributed in the field. It is 
advantageous, too, to employ those rays which make 
as small an angle with the illuminating beam as is 
consistent with other conditions. 
To ensure the dark field it is strictly necessary 
that none of the illuminating light shall, except by 
diffraction, pass into the objective. 
Kirst, we have described in detail the apparatus of 
Siedentopf and Zsigmondy. In this the light from 
a narrow slit is focussed in such a way as to pass 
horizontally through the transparent medium under 
observation, forming a much diminished image of the 
slit exactly in the point of view of the microscope. 
In this image the width of the tape of light pro- 
ducing it corresponds to the length of the slit, and 
the depth to the width of the slit. The depth of the 
illuminated region thus becomes, with a knowledge 
of the diminishing power of the train of lenses, 
strictly calculable, this being of importance in estim- 
ating the number of particles rendered visible in a 
cubic millimetre. No part of the illuminating beam 
can, except when diffracted by small particles, pass 
into the objective. The mean direction of the rays 
which do so pass will be at right angles to the 
illuminating beam. The plan has the great advant- 
age that an immersing liquid can be employed in 
the examination of solids, such as glasses tinted with 
metals, or of liquids beneath a covering glass. The 
adjustments must, however, be extremely nice, and 
require that the whole apparatus should be mounted 
upon one bank. 
The authors have devised a simpler plan of illumin- 
ating the subsurface regions of a medium by taking 
care that incidence with the surface shall be at an 
angle exceeding the critical angle. To this end a 
small but intense beam of light is brought from a 
small arc downwards at an angle of 51° to the 
vertical. This passes at vertical incidence through 
| the bevelled edge of a glass plate about 1 cm. thick 
upon the microscope stage. It is then totally reflected 
upwards by the lower surface towards the upper one. 
Z 
