108 ELEMENTARY CHEMICAL MICROSCOPY 



in that the path of the illuminating rays entering the object cell 

 is at right angles to the optic axis of the observing microscope; 

 but it must be remembered that owing to internal reflections 

 and the impossibility of obtaining a perfectly black background 

 the field is never sufficiently black to render very feeble diffrac- 

 tion evident. This failure to obtain a black background is due, 

 as first stated, to internal reflection on the one hand and upon 

 the other to the fact that the beam of light entering the cell is 

 usually of such a diameter that when the objective is focused 

 upon it there is always a plane below that in focus which contains 

 bright particles. Moreover, this trouble is aggravated for the 

 reason that it is essential to use objectives of long working dis- 

 tance and great penetrating power. These difficulties are largely 

 eliminated in the more recently perfected ultracondensers of the 

 dark-ground illuminator types, since in these devices not only is 

 the background blacker but the light entering the liquid under 

 observation is greater in quantity. For example, in the cardioid 

 condenser, 1 the makers estimate that its light-concentrating power 

 is approximately twenty times that of the slit ultramicroscope. 



In spite of this advantage of the ultracondenser to demon- 

 strate the presence of particles in suspension greatly beyond the 

 limit of instruments of the slit type, preference should be given 

 to the latter form for general use in the chemical laboratory when 

 only a single type of instrument can be purchased, because of the 

 fact that the slit microscope is universal in its application, serving 

 equally well for solids, liquids, gases or vapors, and for hot or 

 cold preparations, while the reflecting condenser types are con- 

 fined to the study of thin films of liquid at room temperature (or 

 in certain restricted cases to the study of tiny transparent fibers). 2 



In all investigations involving quantitative measurements of 

 dispersed phases the slit-ultramicroscope must be employed. 



In instruments of this type the volume occupied by the illumi- 

 nating beam is easily computed. In order that this may be 

 accomplished, the adjustable slit is so mounted that it may be 

 turned through a vertical angle of 90. The diameter of the 

 beam of light in the cell is ascertained by means of an eyepiece 



1 Made by Carl Zeiss, Jena. 2 Gaidukov, Zeit. angevv. Chem., 21, 1 (1908), 393. 



