ILLUMINATION OF OBJECTS; DARK FIELD 45 



the objective which is not corrected for the thickness of the 

 cover of the cell being used. 



A powerful source of light is essential. Direct sunlight by 

 means of a clockwork heliostat is ideal but seldom available. 

 The next choice is 'an electric arc of 4 to 5 amperes or more, for 

 ordinary dark-field examinations, and of 15 to 20 amperes for 

 ultramicroscopic studies of colloids, etc. Useful types of radi- 

 ants will be found described on page 163! 



The more powerful the radiant the smaller the particles which 

 can be demonstrated. Siedentopf estimates that direct sunlight 

 will reveal the presence of particles whose diameters are one- 

 thirtieth of that of the smallest appreciable with the ordinary 

 arc lamp. 



Since the light rays enter these reflecting condensers through 

 an annular space, there being an opaque stop at the center, it 

 is obvious that the spot of light reflected from the mirror of the 

 microscope must have a diameter slightly greater than this 

 space, otherwise the illuminator will not properly function; for, 

 this reason, before placing the illuminator in position for cen- 

 tering, it is always essential to examine its lower surface and 

 ascertain the diameter of the spot of light necessary to com- 

 pletely fill the annular entrance space. The radiant and a suit- 

 able condensing lens are then so placed as to yield parallel rays 

 and produce a spot of light of the proper size and intensity at 

 the center of the plane mirror of the microscope, the mirror being 

 so inclined as to reflect the light rays into the dark-field illu- 

 minator. Dark-field illuminators require that an immersion 

 fluid be placed between them and the object slide. To obtain 

 the best results homogenous immersion oil should be employed, 

 water seldom yields good results. 



In applying the immersion fluid and laying the object slide 

 in place great care must be taken to prevent the entrance of 

 air bubbles or dust particles. 



Because the light rays are caused to emerge from the illumina- 

 tor at such an angle (determined by the inclination of the reflect- 



1 The " Chalet Lamp " of the Bausch & Lomb Optical Co. is of especial con- 

 venience and value with dark-field illuminators. 



