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,^ 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) ? 
In all investigations involving quantitative measurements of 
dispersed phases the sHt-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 
* Made by Carl Zeiss, Jena. ^ Gaidukov, Zeit. angevv. Chem., 21 ,1 (1908), 393. 
