CHAPTER V 

 ULTRAMICRO SCOPES. 



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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