86 SCIENCE PROGRESS 



Our knowledge of the form of the ultramicroscopic particles 

 depends on the polarisation effects of them. The light which 

 is diffracted by the granules is polarised to some extent, and 

 on the amount of this polarisation and the direction of its 

 plane depends the appearance of the particles. These two 

 quantities are determined by the shape and position of the 

 object. The theory of the methods and the experimental 

 results obtained are somewhat complicated, but it may be 

 stated that Siedentopf, Garnett, and others find that in the 

 case of films coloured by gold the particles act as though they 

 were spherical. Those occurring in crystals of rock-salt, in 

 colloidal liquids, and large particles in general, appear to be 

 in the form of needles. This explains the scintillations of the 

 particles in a colloidal liquid. As a result of the Brownian 

 movements the orientation of the particle is constantly changing, 

 and the intensity of the light diffracted in a given direction 

 is thus changed. This would give rise to scintillation. 



Again, just as glass can be made doubly refracting to light 

 by straining (Kerr effect), so colloidal liquids can be caused 

 to become birefrigent and dichroic by exposure to a transverse 

 magnetic field. When certain colloidal liquids are placed in 

 a strong magnetic field and observed in a direction at right 

 angles to this field they act as birefrigent substances, the 

 velocity of light through it depending on the direction of the 

 vibrations in the beam. This phenomenon can be more readily 

 understood if the particles are elongated, and are orientated 

 under the influence of the field. 



We have confined ourselves chiefly to the study of solid 

 substances and colloidal liquids, and it is perhaps in these 

 physical and chemical branches that the ultramicroscope has 

 been of most service. But there are many other directions 

 in which it has been applied. Aggazzotti and others have 

 employed it in the study of fibres, living cells, and bacteria; 

 but the advance made here is, as yet, not so great, and reference 

 must be made to the original papers for the details of the work. 



The use of the ultramicroscope and of dark ground illumina- 

 tion is as yet in its infancy. In the time that has elapsed great 

 strides have been made, and it may confidently be expected 

 that in the near future it will be of further service, not only 

 in the interests of pure science, but also in its applications, 

 and so more directly to humanity. 



