ULTRAMICROSCOPES 57 



Ultramicroscope of Siedentopf and Zsigmondy. At first sight 

 this instrument might be thought to be also the most efficient, 

 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 con- 

 tains bright particles. Moreover, this trouble is aggravated for 

 the reason that it is essential to use objectives of long working 

 distance 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-concen- 

 trating 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 confined to the study of thin films of liquid at room tempera- 

 ture (or in certain restricted cases to the study of tiny transparent 

 fibers). 2 



The Slit Ultramicroscope consists of an ordinary compound 

 microscope, a special cell of black glass with small windows at 



1 Made by Carl Zeiss, Jena. 



8 Gaidukov, Zeit. angw. Chem., 21, 1 (1908), 393. 



