ULTRAMICROSCOPES 107 



The amplitude of the Brownian movement may be ascertained 

 by means of a net ruled eyepiece micrometer calibrated in the usual 

 manner. Space forbids a discussion of the experimental details. 1 



The light emanating from the particles is polarized, the inten- 

 sity of polarization increasing with the decreasing size of the 

 particles. This fact enables us to differentiate between light 

 diffracted by the particles and light emanating from fluorescent 

 bodies, since fluorescent light is not polarized. A well-equipped 

 ultramicroscope must therefore include a device for the pro- 

 jecting of polarized light into the preparations and an analyzer 

 for the study of the light rays forming the image in the micro- 

 scope. But it must be remembered that even in the highest 

 developed types of the ultramicroscope tiny particles in suspen- 

 sion are discernible only when the refractive indices of these 

 particles are different from that of the medium in which they 

 are suspended; otherwise, no light will be diffracted from them. 

 Therefore, although a medium may appear to be " optically 

 empty " when viewed in the ultramicroscope, it by no means 

 follows that there are no so-called " colloids " in suspension. 

 To meet this difficulty and to extend the range of the ultramicro- 

 scope, W. Ostwald 2 has suggested that monochromatic light be 

 employed. This suggestion is based upon the fact that although 

 two substances may have an identical value for their refractive 

 indices for white light, with light rays of certain definite wave- 

 length the indices may be sufficiently different to permit the 

 illuminating rays to render the tiny particles manifest. 



To the smallest particles visible in the ultramicroscope the 

 terms micellae, ultramicrons or submicrons are sometimes given. 

 Particles still smaller and therefore invisible in the ultramicro- 

 scope are called amicrons. , 



The earliest practical instrument may be said to be the Slit 

 Ultramicroscope of Siedentopf and Zsigmondy. At first sight 

 this instrument might be thought to be also the most efficient, 



1 For further details relative to the Brownian movement the student should 

 consult: Perrin, C. r. 146 (1908) 967. Rutherford, Science 30 (1909) 289. Fletcher, 

 Phys. Rev. 33 (1911) 81. 



2 Ostwald, W., Zeit. f. Ind. Kol., 11 (1912), 290. 



