NEW MICROSCOPES—SEIDEL AND WINTER 201 
stance, let us suppose an object is examined through which run two 
very fine parallel lines closely set together. If the two lines are visible 
under the microscope and are revealed as two separate images, then, 
apparently, no limit of resolution has been reached; but if the two 
lines are merged or revealed as only one, and upon further magnifica- 
tion the image merely becomes enlarged without separation of the 
lines, then a limit of resolution apparently has been reached and ad- 
ditional magnification would constitute only enlargement. Assum- 
ing now that the object is a point object in which case the images of 
the points would be diffraction disks, the disks should likewise be 
sufficiently resolved so that each may be distinguished as a single 
image. If, when these disks are seen to overlap, additional magnifica- 
tion fails to extend the distance between them, their size simply in- 
creasing in proportion to the increase of magnification, or, if they are 
all but completely merged and the image becomes just a spurious disk 
of light, it is evident that a definite limit of resolution has been at- 
tained and that further magnification would be useless. Resolution, 
in a broad sense, then, is the ability of the microscope to bring out or 
reveal internal structure and detail of a specimen, the shortest dis- 
tance it is possible to separate two component parts, according to 
Abbe, being not less than the wave length of light by which the 
specimen is illuminated divided by the numerical aperture of the 
objective lens plus the numerical aperture of the condenser lens, or 
about one-third the wave length of light utilized. 
The several factors which are generally acknowledged to be re- 
sponsible for the limitation of resolving power are interrelated. Now 
when light passes from one medium into another of different density— 
in the instance which we are considering that of light refracted by the 
specimen and passing from air into glass—the light rays are deviated 
from their straight-line course; that is to say, when they come to 
within a very short distance of this denser medium, they are acted upon 
by a very powerful force in such a manner that they execute a short, 
rapidly curving motion, or an angle, and are pulled into the medium of 
greater density. When the rays of light undergo such a force, the 
momentum of the corpuscles is increased and the speed of the waves 
decreased, resulting, of course, in a shortening of the wave lengths. 
Here, again, we may make use of the second of the rules of correlation— 
“Momentum (of corpuscles) varies inversely as wavelength (of 
waves).” Once well inside the new medium, however, the light rays 
straighten themselves out again (unless the medium is so constructed 
that it possesses gradation of density, in which case they follow a curved 
path). They do this in spite of the fact that the same forces are still 
acting upon them, although now these forces issue from all sides of 
them and so cancel each other out, the momentum of the photons or 
