CH. II] 



DARK-GROUND ILLUMINATION 



FIG. 47. REFRACTING OR 

 BRIGHT-FIELD CONDENSER 

 WITH CENTRAL - STOP TO GIVE 

 DARK-FIELD ILLUMINATION. 



The condenser is shown in 



Open the substage diaphragm to its full 

 extent and reflect the light up through 

 the condenser. The particles will now 

 appear dark on a white field. Make the 

 light unsymmetrical and very oblique 

 by putting the finger in the path of the 

 light on one side (fig. 46) and the field will 

 be dark and the particles bright. 



122. By lighting with a hollow 

 cone. This lights the object with a ring 

 of light, all the rays of which are so 

 oblique that they fall outside the objec- 

 tive (fig. 47). Some of this very oblique 

 light is deflected by the object into the 

 microscope objective, and thence passes 

 to the eye. The question naturally arises 

 how one is to determine the size of the 



central Stop to use with a given con- and continuing wholly outside 

 denser and objective. This is easily 

 determined as follows : The field is lighted 

 well as for ordinary bright-field observa- 

 tion, and the microscope is then focused 

 on some object. The object is then 

 removed and the iris diaphragm of the 

 condenser opened to its fullest extent. 

 If one then removes the ocular and looks 

 down the tube of the microscope the back 

 lens will be seen to be fully illuminated 

 (fig. 456). If now the iris diaphragm is 

 slowly closed it will be seen to reach the 

 margin of the back lens, and if closed still 

 farther the light will be cut out around 

 the margin. The, size of the central stop 

 to use is the opening in the iris diaphragm 

 when the back lens is just filled with light. This opening in the 

 iris can be measured by dividers, and then a central stop prepared 



The light deflected by the ob- 

 ject into the objective is shown 

 by broken lines. 



Axis The principal optic 

 axis of the condenser and of 

 the microscope. 



CS Central stop to cut 

 out the middle rays of the solid 

 cone of light. Above it is 

 shown in section, and below in 

 face view. 



Glass Slide The slip of glass 



Tn which the object is mount- 

 It is advantageous to have 

 the slide in homogeneous im- 

 mersion contact with the top of 

 the condenser ( 126). 



Object The object to be 

 studied by dark-field illumina- 

 tion. 



Objective The front lens of 

 the microscope objective. 



