ON THE ILIaUMINATION OF OBJECTS. 



195 



presents the stage of the microscope in an inclined position ; 

 c a condensing lens placed below and on one side of the 

 stage ; d a lamp also placed below the stage and at some dis- 

 tance from it; the rays of light rri^r emanating from the lamp. 



Fig. 123. 



are thrown obliquely upon the object e, and only those that 

 are refracted by the glass, or by the object under examination, 

 will pass through the compound body to the eye: some objects, 

 when viewed in this way, will appear beautifully illuminated 

 on a field of view that is nearly, if not quite, dark. This is a 

 valuable method of illumination for some preparations, but 

 not for others ; hence, in all newly investigated structures, it 

 should be employed to see what effect it produces on them. 

 It is also capable of being accomplished by dayHght by elevat- 

 ing the condenser c above the stage, and then allowing the 

 rays from a white cloud, if possible, to pass through it ; but 

 the effect is not quite so striking as when a kmp is employed, 

 unless the eyes be shielded by the hood or bonnet, described at 

 page 147, from all the surrounding light. These different 

 modes of illuminating transparent objects will be again alluded 

 to in the chapter devoted to test objects. 

 13* 



