36 
ELEMENTARY CHEMICAL MICROSCOPY 
true morphology may be more easily discerned. Usually a 
long exposure is made by oblique reflected light or by means of a 
Silvermann illuminator using a suitable background^ and a second 
short exposure is then made by transmitted light. Fig. m 
gives a fair idea of what may be gained through this procedure. 
But it is not only in photography that dual illumination 
is of value. In ordinary routine industrial microscopy the author 
finds that he has occasion to employ it constantly as an aid to 
the interpretation of appearances and also to render the study 
of certain preparations less fatiguing. When examining per¬ 
fectly opaque material in coarsely granular form using oblique 
rays for illumination, it will be found that the admission of a 
very little transmitted light will aid greatly in bringing out 
the form of the particles. Too much transmitted light will 
completely spoil the effect. 
e. Dark-field Illumination as opposed to bright-field illumi¬ 
nation discussed above under sections a and h, is usually obtained 
by sending oblique light rays into the preparation from below, 
at such an angle that no direct rays enter the objective. This 
is accomplished by introducing a metal stop below the Abbe 
condenser so as to shut out all central rays and allow only rays 
near the circumference of the condensing lenses to enter the 
preparation, or, better, by substituting for the Abbe condenser 
a device which will reflect rays from a curved surface in such 
a manner as to bring them approximately to a focus. In prepa¬ 
rations thus illuminated objects appear to be self-luminous 
and are therefore bright upon a black background. 
This method is invaluable for demonstrating the presence of 
very minute bodies or those whose index of refraction is so very 
nearly the same as that of the medium in which they occur as 
to cause them to escape detection when illuminated by trans¬ 
mitted light. 
It is generally the case that particles of a diameter of one mi¬ 
cron or less require dark-field illumination for their demonstration. 
If the obliquity of the rays from the illuminating device is 
very great, the dark-field illuminator becomes an “ ultracon- 
^ See Differential Color Illumination, etc., page 47. 
