154 MICROSCOPIC OBJECTS. 



will answer for temporary purposes ; but I have never 

 found anything equal to bronzing the surface by the 

 solution of platina. Another method of stifling false 

 light is by stops ; these are very useful in the body of a 

 compound microscope. These stops are diaphragms 

 made of plates of metal or wood, having a hole in their 

 centre, and then blackened. Persons should be on their 

 guard in examining an instrument having these stops ; 

 for they are often put in to cut off the aperture of the 

 object-glass, and thereby deceive the inexperienced, who 

 seeing the size of the object-glass imagine the whole is 

 used, whereas the most important portion is not employed. 

 This is a very common case, and ought to be exposed. 

 Diaphragms under the stage improve the definition of a 

 microscope, but reduce the angular aperture, and con- 

 sequently the penetration ; they are nevertheless very 

 useful and necessary for some objects, especially where 

 currents or things in motion are to be examined. 



(2.) Mounting Transparent Objects. — The most usual 

 method of preserving these objects in a dried state is 

 between plates of talc or mica fitted into cells formed in 

 ivory sliders, having a split ring of wire to secure them. 

 The bottom of these cells should be turned quite flat, to 

 afford a good bearing for the mica, and of sufficient 

 thinness to permit the magnifiers to approach the 

 object. In using these slides, it must be observed 

 that there is a wrong and a right side, or the 

 student wiU not be able to approach close enough to the 

 object with the high powers. The ring side' should 



