62 SCIENCE PROGRESS 



present in the air. The drops forming the clouds produced 

 when air saturated with water or other vapour expands can 

 be rendered visible in the same manner by passing a beam 

 of light through the cloud vessel. 



Particles very small compared with the wave-length of 

 the light used can give rise to diffracted light in this way. 

 According to Lord Rayleigh, it is possible that the blue 

 colour of the sky is due to diffraction by the molecules of 

 the air. 



If the intensity of the light diffracted by the particle become 

 too small, the latter will no longer be visible. Since this 

 intensity is proportional to that of the incident light, the im- 

 portance of using a powerful beam, which can be concentrated 

 on the particles, is obvious. The size of the diffracting particle 

 is also important. Lord Rayleigh has shown that the amplitude 

 of the diffracted light is proportional to the volume of the 

 particle ; therefore the intensity of the light diffracted by a 

 particle of volume nv will be n 2 times as great as that diffracted 

 by one of volume v. The smaller the particle, therefore, the 

 greater must be the intensity of the incident light. Cotton 

 and Mouton have found that certain particles are only visible 

 when sunlight is used in the middle of a summer day. 1 



The importance of using a dark background has been 

 pointed out above ; and in order to achieve this, it is essential 

 that none of the light of the illuminating beam should enter 

 the microscope and that all stray light be excluded. It will 

 be advantageous also to illuminate only those particles which 

 are to be examined ; any other particles which may be 

 illuminated will scatter the light, thus reducing the contrast 

 between the particles and the background. 



Finally, if the particles are to be seen to the best advantage, 

 too many must not be illuminated. 



Necessary Conditions. — Thus there are two essential con- 

 ditions, which must be fulfilled if the ultramicroscopic particles 

 are to be seen to any advantage, or even at all : (i) a powerful 

 beam of light must be concentrated on them, care being taken 

 that the only light entering the microscope is the diffracted 

 light; (2) the particles must not be too close together. 



Apparatus of Siedentopf and Szigmondy— The earliest form 



1 The intensity of bright sunlight is about ten times that of a powerful arc 

 lamp. 



