PEELIMINAEY DIRECTIONS. 185 



the image of the flame may be seen co-incident with the 

 object. 



Opaque Objects. — These are illuminated either by the bull's- 

 eye condensing lens, the Lieberkuhn, or the side reflector, and 

 the object-glasses employed cannot exceed the one-fourth of 

 an inch, as no higher magnifier is provided with a Lieberkuhn, 

 and the light from the condensing lens or side reflector cannot 

 well be thrown upon any opaque object having the object-glass 

 nearly approaching it. Opaque objects are generally mounted, 

 either on slips of glass or on discs of card or other material, 

 having a pin passed through them ; these require to be held 

 in the forceps, described at page 129; and if the Lieberkuhn 

 be employed, the diaphragm plate under the stage must be 

 removed, and the light thrown upon the Lieberkuhn by the 

 concave mirror in the same manner as was described for a 

 transparent object, the lamp or other Uluminating agent being 

 placed either in front or on one side of the mirror. If the 

 microscope be inclined, and it be required to illuminate an 

 opaque object by the condensing lens, the lamp must then be 

 brought near to the eye of the observer, and the condenser 

 placed between it and the object ; the distance between the 

 lamp and the condenser being in aU cases greater than that 

 between the condenser and the object, otherwise the rays of 

 light will not be made to converge upon the object. If the 

 disc or glass on which the object is mounted, or the object 

 itself, be not large enough to cut off the light from entering 

 the object-glass, then a dark stop or well, represented by fig. 

 79, must be employed to form a back ground. 



