56 ELEMENTARY CHEMICAL MICROSCOPY 



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. 



(Note. The student should read the excellent summary of 

 the then known facts relative to the Brownian movement given 

 by Rutherford in Science, 30 (1909), 289-302.) 



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

 tensity 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 flu- 

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

 equipped ultramicroscope must therefore include a device for 

 the projecting of polarized light into the preparations and an 

 analyzer for the study of the light rays forming the image in the 

 microscope. 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 ultrami- 

 croscope, W. Ostwald 1 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 ultra-micro- 

 scope are called amicrons. 



The earliest practical instrument may be said to be the Slit 

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



