ULTRAMICROSCOPES 55 



particles enters the microscope and eventually the eye of the 

 observer, and that therefore he never sees the particles them- 

 selves, but merely a diffraction disk of light. We know of the 

 existence of these particles through the same manifestation of 

 more or less scintillating points of light that we see in the fixed 

 stars on a moonless night. As hereinbefore stated the image 

 of a point of light is a diffraction disk surrounded by alternate 

 dark and bright rings. These diffraction disks appear to be 

 in rapid motion. They appear to spin, to expand or contract 

 and are endowed with a constant vibratory movement. This 

 is due to the fact that exceedingly minute particles suspended 

 in a liquid exhibit a constant vibratory and rotatory motion, 

 long called the Brownian movement and now known to be as- 

 sociated with and a manifestation of what we commonly term 

 molecular vibration or bombardment. The presence of disin- 

 tegrating or so-called " digestive " colloids increases the Brownian 

 motion, while electrolytes by reason of their causing agglutina- 

 tion tend to decrease the amplitude of the paths of vibration. 



In the few years that ultramicroscopic research has become 

 possible a large number of investigations have been made upon 

 the amplitude of the paths of vibration of the finest of these 

 infinitely small suspended particles, with the result that the 

 measurements made agree very closely with the theoretical 

 values computed for the amplitudes of vibration of the molecules. 

 Agencies which increase molecular vibration, such as heat, 

 dilution and consequent reduction of viscosity, increase the 

 Brownian movement. Hence, we find under the ultramicroscope 

 the suspended particles in a gas (as, for example, in smoke) 

 in much more rapid motion than in a liquid, while in a solid the 

 Brownian movement is visible only with the greatest difficulty. 



Since the tiny particles in suspension are being bombarded 

 on all sides, the motion imparted to them must be the resultant 

 of the forces acting ; we therefore find them spinning rapidly as 

 well as moving to and fro. Some authors have even suggested 

 that the term kryptokinetic motion be assigned to the rotatory 

 movement to distinguish it from the oscillating Brownian vi- 

 bration. 



