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 1 and a second 

 short exposure is then made by transmitted light. Fig. in 

 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. 



c. Dark-field Illumination as opposed to bright-field illumi- 

 nation discussed above under sections a and b, 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- 



1 See Differential Color Illumination, etc., page 47. 



