CHAPTER V 
ULTRAMICROSCOPES. 
APPARATUS FOR THE STUDY OF ULTRAMICROSCOPIC 
PARTICLES. 
Ultramicroscopes. — Attention has already been called to 
the fact that the compound microscope with transmitted axial 
light will resolve tiny particles in suspension in a liquid only 
when there is a certain appreciable difference between the re¬ 
fractive index of the particles and that of the liquid, and when 
the diameters of the particles are greater than half the value as¬ 
signed to the shortest wave lengths producing the effect of light 
upon the normal human eye. We have also seen that if instead 
of axial light, oblique rays are employed the ability to discern 
minute particles and intricate structure is greatly increased, 
especially if the obliquity of the rays is such as to yield an illu¬ 
minated object upon a black background. If the degree of 
inclination of the illuminating rays be still further increased 
and the source of the rays a powerful radiant and the objective 
employed one of low numerical aperture, only light diffracted by 
the object will enter the objective; the phenomenon known as 
the “Tyndall effect” results, so familiar in the scintillating dust 
particles visible when a ray of sunshine enters a tiny opening 
in a darkened room or cell. The existence of these infinitely 
minute particles in suspension in the air is manifest to the naked 
eye through that phenomenon, although even a high-power 
microscope fails to resolve them. The ultramicroscope is merely 
the adaptation of this Tyndall effect to microscopic illumination. 
As a result, the existence may be demonstrated of particles 
almost one thousand times smaller than is possible by means of 
the most powerful instrument employed in the usual manner. 
It is obvious that under the illumination of these very oblique 
rays, light alone which has been diffracted or reflected by the 
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