OBJECTIVES AND OCULARS 
17 
The shortest violet rays producing the effect of light upon the 
average normal human eye may be assumed to have a wave 
length of approximately X 4000 (or 0.4 /4)b It has been shown 
that under ordinary conditions the smallest particle which will 
be visible as a black spot upon a light ground must have a diam¬ 
eter equal to at least half this value (Helmholtz-Abbe). More¬ 
over, a lens, owing to diffraction, yields as an image of a point, 
a diffraction disk and not a point. The final image may be con¬ 
sidered as consisting of a series of diffraction disks or patterns, 
and if the distances between bright points are such as to cause 
an overlapping of the resulting disks or their surrounding circles, 
a blurring of the image must result. Thus we are limited, in 
our attempt to see and study infinitely small particles, by the 
sensitiveness of the human eye, on the one hand, which cannot 
properly respond to the stimuli of very short wave-lengths, and 
to the fact, on the other hand, that no matter how great the 
magnification employed we cannot bring about a separation of 
the overlapping rings of the diffraction patterns. The result, 
therefore, must be at the best a vague, blurred, uninterpretable 
image or merely a diffraction pattern. 
If, therefore, our wave theory of light is correct, the most 
minute particle which we may hope to render distinctly visible 
by our compound microscopes by transmitted light must have 
dimensions of at least 0.2 /x. It should not be inferred, however, 
that the existence of particles many times smaller cannot be 
indicated, for an invisible particle may yield a large diffraction 
pattern, a phenomenon which makes ultra-microscopic investi¬ 
gations possible; but we must bear in mind that in the case of 
ultra-microscopic particles we have no picture or image of their 
shape or structure and that we know of their existence simply 
through the light diffracted by them and thus have passed far 
beyond the range of the resolving power of our lenses. Although 
it is true that the limit of resolving power, 0.2/x, has been seri¬ 
ously questioned by men of recognized authority, it may be 
accepted as beyond dispute that a moderately skillful micros- 
^ One micron, designated by the Greek letter /x, is equivalent to one-thousandth 
of a millimeter (0.001 mm.). 
