ILLUMINATION OF OBJECTS; DARK FIELD 37 



denser " and may be employed for demonstrating the presence 

 of particles less than 0.2 ^ in size. 



Dark-field illumination is employed in practice in the exami- 

 nation 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 crys- 

 tals, submicroscopic particles, colloids, etc. 



If the Abbe condenser is to be employed for dark-field illu- 

 mination, insert one of the dark ground stops in the ring attached 

 to the bottom of the condenser mounting, open the iris dia- 

 phragm 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 between 

 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- 

 ence. Special dark-field illuminators are marked by the manu- 

 facturers with the thickness of object slide for which they are 

 designed. 



The use of the Abbe condenser with dark-field stop as a sub- 

 stitute for special dark-field illuminators is not to be recom- 

 mended since the obliquity of the rays is seldom sufficient to 

 prevent some light from entering the objective. The results 

 usually obtained are poor and unsatisfactory. 



Dark-field Illuminators are condensers of such construction 

 that very oblique light rays are caused to converge, usually by 

 reflection. 



The rays either pass through the preparation at an angle 

 with the perpendicular so great that they fail to enter the objec- 

 tive (providing it is of low numerical aperture) or they strike 

 the cover glass at such an angle as to be reflected downward 

 and therefore fail to enter the objective. When no object lies 

 in the field and no fine particles occur in the mounting medium 

 between slide and cover glass, the field of the microscope is 

 uniformly dark. In order that there may be no change in 

 direction (through refraction) of the rays emerging from the 



