BANISHING THE PLAGUES 



into the otherwise totally dark field of the microscope 

 they will appear as sparks of light. It has been found 

 possible by this method to reveal the presence of 

 particles that are estimated to be not more than ten 

 times the size of a molecule. Such particles lie far 

 beyond the limits of direct vision even under the 

 most powerful magnification. 



The effect will be understood if we recall the famil- 

 iar observation that a beam of light penetrating a 

 dark room through a hole in the shutter, reveals 

 the presence of myriads of motes, floating in the 

 atmosphere, which disappear instantly when they 

 float outside the beam of light. This method was 

 used by Professor Tyndall to test the presence of 

 bacteria in the air, and he found that the naked eye, 

 with the aid of the sunbeam, could detect the presence 

 of spores that otherwise would be invisible except 

 with the aid of a microscope magnifying perhaps a 

 thousand diameters. 



The beam of light passing through the field of the 

 ultra microscope, since it is viewed through a mag- 

 nifying lens, reveals particles infinitely more minute. 

 These exceedingly fine particles are thus observed 

 to be incessantly dancing about in a zig zag motion 

 which would be quite inexplicable except on the sup- 

 position that they are being buffeted by the invisible 

 molecules of the solution in which they are found. 

 The degree of activity of ultra-microscopic particles 

 varies, as might be expected, with the strength of 

 the solution; which is in harmony with Van't Hoff's 

 theory of osmosis detailed elsewhere in this volume. 

 It remains to be demonstrated whether the ultra- 

 16 233 



