196 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
is formed. Finally, the projection coil, corresponding to the projec- 
tion lens or ocular, produces a further magnified image on a large 
fluorescent screen. In some of the electron microscopes, there is a 
periscope-like attachment by means of which it is possible to locate 
and adjust for study the most interesting portion of the specimen, or 
that which it is desired should be examined, before the projection lens 
coil forms the final magnified image upon the screen, since it is some- 
times difficult to accomplish this at high magnification. Also, if it 
is desired that a photographic record be made, the screen can be 
removed and a photographic plate substituted. 
The specimen itself is supported on a thin nitrocellulose membrane 
less than one-millionth of an inch thick, and clamped in the tip of a 
cartridge which is inserted between the pole pieces of the objective coil. 
The membrane is suspended across the opening of a fine-mesh screen, 
and a plate, serving as the movable stage, supports the cartridge. The 
image is projected onto the screen according to the density and atomic 
weight of the specimen. In other words, whereas in the ordinary light 
microscope the image is seen because of refraction of the specimen or 
differences in absorption, in the electron microscope the image is seen 
through scattering of the electrons, and since electrons travel in a 
straight line in a vacuum, it stands to reason that even a fairly thin 
specimen will prove sufficient to deflect such particles. Electrons 
which strike a thick or solid portion of the specimen will, of course, 
not continue on in a straight line to the screen but will be either com- 
pletely absorbed by the specimen or scattered too far out of the beam, 
thus failing to enter the narrow aperture of the objective, so that that 
portion of the screen corresponding to the thick portion of the specimen 
will remain dark. However, those electrons which are able to escape 
complete absorption or too great deflection, because they do not happen 
to come in contact with too solid a portion of the specimen and either 
pass along on all sides of it or penetrate the thinner portions where it 
is possible they may encounter only a single heavy nucleus for consider- 
able scattering (the angle of deflection being proportional to the 
square root of the thickness), continue on to the screen where they im- 
pinge and cause the chemically treated screen to fluoresce, thus provid- 
ing a study in light and shadow. If the atoms of a particular sub- 
stance are heavy, they will also deflect more electrons than if they were 
light. It may be readily seen, therefore, that the thinner the specimen 
and its mounting, or the greater the variations in density of the speci- 
men, the more internal structure and detail which may be seen, since 
too great density tends to absorb or interrupt the straight-line progress 
of too many of the electrons. 
Focusing of the image is accomplished by varying the strength 
of the fields and thereby altering the focal length of the “lens” coils at 
