METHODS OF COLLOID RESEARCH 121 



solutions, but not for those deeply colored. It depends on the follow- 

 ing principle : when parallel beams of light pass through a narrow slit, 

 as the result of refraction a broad band of light with parallel dark 

 bands (interference bands) is seen on the opposite wall. If light is 

 permitted to fall on the same spot through a second parallel slit, the 

 bands of light will interfere and very fine sharp lines will be obtained 

 which may be greatly magnified. When a different medium, that is 

 water or a solution of salt or colloid is 

 placed behind one slit the interference 

 bands move to one side depending on 

 the refractive index. If the process is 

 reversed by a set of glass prisms or 

 something similar, it is possible to read 

 on the adjusting screw of the appa- 

 ratus the difference of refractive index. 

 Fig. 28b shows a cross section 



through the interferometer. A is the . 



, _ _ FIG. 28b. Fluid Interferometer, 



chamber with the standard water, B 



the chamber for the test solution, C the window for viewing the 

 interference bands. With dilute solutions the concentration increases 

 in proportion to the scale on the graduated drum. For more concen- 

 trated solutions a standard must be set in each case. Technical de- 

 tails of the readings may be found in MARC'S paper (loc. cit.). He 

 has thus far used the interferometer mainly to determine adsorption 

 and for studying the colloid content in drinking water and sewage. 



Ultramicroscopy. 



Ultramicroscopy permits the recognition of certain optical in- 

 homogeneities, and depends upon the use of dark field illumination. 

 Ultramicroscopes magnifying from 750 to 1500 diameters serve in 

 principle the same purpose as the ordinary microscope. They have 

 the advantage over the latter that without staining or extensive 

 preparation, even living objects, spirilla, etc., become visible to the 

 eye; bright on a dark background. Ultramicroscopy with a one hun- 

 dred thousand fold magnification has solved important theoretical 

 questions of colloid chemistry. By reason of the conditions of 

 light refraction its value is chiefly confined to inorganic colloids. 



In the ordinary microscope the field is usually bright, while the 

 object is more or less dark against its surroundings. In the ultra- 

 microscope, only the rays of light reflected from the object reach the 

 observer's eye and permit the object to stand out bright against the 

 dark background. 



