OPAQUE OBJECTS. 



201 



is therefore produced, and a correct figure obtained. If the 

 object be a transparent one, it is necessary that some kind of 

 dark ground be placed at the back of it, to prevent the central 





Fig. 128. Fig. 129. 



rays of light from passing to the object-glass : this may be 

 effected by the employment of one or other of the dark stops 

 or chambers, represented by fig. 130, which should be brought 

 into the axis of the instrument, so as to cut off all the rays 

 that otherwise would pass to the object-glass, and so inter- 

 fere with the definition, as it is highly necessary that no rays 

 should pass to the magnifier, save those from the object itself. 

 These stops are made of different sizes for the different 

 magnifying powers ; the lower the power the larger the stop 

 required. When the stops are not fitted to a microscope, 

 their place may be supplied by a disc of black paper laid 

 behind the object ; or if the preparations be mounted either 

 on discs of cork or in cells, as hereafter to be described, the 

 use of the dark stop may be dispensed with. Many persons 

 prefer mounting their opaque objects on papers of different 



