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.^ 
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 hght 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 ^ 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 ami crons. 
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 (iqo8) 967. Rutherford, Science 30 (1909) 289. Fletcher, 
Phys. Rev. 33 (1911) 81. 
2 Ostwald, W., Zeit. f. Ind. Kol., 11 (1912), 290, 
