METHODS OF COLLOID RESEARCH 



125 



There are a number of precautions to be observed in working with the ultra- 

 microscope. Since every impurity makes a point of light in the field, it is neces- 

 sary to employ optically clear water. Such water is prepared according to R. 

 ZSIGMONDY by distillation through a silver condensing tube, or according to H. 

 BECHHOLD by ultrafiltration through a very tight ultrafilter (6 to 10 per cent). 



FIG. 34. Dark-field illumination for 

 the examination of organisms. 



For collecting and storing, only Jena glass vessels should be employed. Ground 

 glass stoppers or corks are to be avoided because they always yield fine dust. 

 I have found the suggestion of W. BILTZ serviceable ; he coated the stoppers with 



FIG. 34a. 



tin foil. I recommend a storage flask for ultrawater manufactured by A. HAAK 

 in Jena (Fig. 33). 



Neither water nor alcohol should show microscopically the slightest FARADAY- 

 TYNDALL effect, but wherever illuminated, only a very faint shimmer, whitish 

 in the case of water (ultrawater), bluish in the case of alcohol (ultra-alcohol). 



