NEW MICROSCOPES—SEIDEL AND WINTER 197 
will, so that the need of changing the specimen’s position in relation 
to a fixed optical system, as would be the case with an ordinary light 
microscope, is avoided. Thus, magnification in an electron micro- 
scope can be continuously varied. 
Some specimens may be mounted directly on the fine-mesh screen 
while others may be embedded in collodion, sealed between films of 
collodion, or suspended in a gelatin film, itself supported on collodion 
film. The supporting films beside being very thin must be homo- 
geneous lest an artifact be created. For the most part, no staining 
of bacteriological specimens is done since usually they exhibit suffi- 
ciently high contrast in density to reveal readily flagella and other de- 
tail without any preparation except that of suspending the specimen in 
distilled water or other liquid and allowing a drop of the suspension 
to dry on the film surface, which method is also utilized for specimens 
of colloidal particles, pigments, and other chemical preparations. At 
times, however, as Dr. L. Marton, of Stanford University, has men- 
tioned in his article on the electron microscope (written for The Jour- 
nal of Bacteriology, March 1941, when he was associated with the 
R. C. A. Research Laboratories), virus particles may show decided low 
contrast. One method which Dr. Marton mentioned for overcoming 
this is to obtain a number of electron micrographs at various focuses 
and simply select the best one for study. Or the virus may be per- 
mitted to absorb colloidal gold which would result in an image of high 
contrast. Dr. Marton points out that there may be future need for a 
staining in density and that already osmic acid has oe tried and used 
for elite’ purpose. 
In this microscope, voltages of between 30,000 and 60,000 are used. 
It has been previously stated that the higher the voltage, the greater 
the speed of the electrons. This might now be augmented to read, 
the higher the voltage, the greater the speed of electrons; hence, the 
shorter the wave length. An explanation of this may be approached 
through a brief discussion of short-wave diffraction as considered by 
Dr. Karl K. Darrow, of Bell Laboratories, in his book, “The Renais- 
sance of Physics.” In order to obtain convenient angles of refraction 
with the ordinary diffraction grating, it is necessary that the wave 
lengths of light be smaller, but not many times smaller, than the spac- 
ing between the wires or grooves. Naturally, a limit of measurement 
is reached in the region of ultraviolet light since it is impossible to 
lessen further the spacing of these gratings. However, this limitation 
was overcome when von Laue conceived the idea of substituting a 
erystal for an artificial grating since the atoms in a crystal are a 
thousand times more closely set together than are the wires or grooves 
of a grating and are arranged in precise regular order or “lattices,” 
and, like gratings, are unable to diffract waves which are longer than 
