40 ELEMENTARY CHEMICAL MICROSCOPY 



e. Dark-ground Illumination is usually obtained by sending 

 oblique light rays into the preparation from below, at such an 

 angle that no rays directly enter the objective. This is accom- 

 plished by introducing a metal stop below the Abbe condenser so 

 as to shut out all central rays and allow only rays near the cir- 

 cumference 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 preparations thus 

 illuminated objects appear 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 

 micron or less require dark-ground illumination for their demon- 

 stration. 



If the obliquity of the rays from the illuminating device is 

 very great, the dark-ground illuminator becomes an " ultra- 

 condenser " and may be employed for demonstrating the presence 

 of particles less than 0.2 /i in size. 



Dark-ground illumination is employed in practice in the ex- 

 amination of blood for the presence of parasitic organisms, in 

 the study of bacteria, in the biological examination of water, 

 in the study of foods, fibers, crystallization phenomena, tiny 

 crystals, submicroscopic particles, colloids, etc. 



If the Abbe condenser is to be employed for dark-ground 

 illumination, insert one of the dark ground stops in the ring 

 attached to the bottom of the condenser mounting, open the 

 iris diaphragm to its full capacity, and screw up the condenser 

 in its mounting until, when turned in place and the substage 

 is racked up to its highest point, the upper lens will just touch a 

 slide laid upon the stage. A drop of water is then placed be- 

 tween the condenser lens and the preparation to be examined. 

 It is always essential to ascertain the thickness of object slides 

 which yield the best results and keep this value for future refer- 



