OF THE MICKOSCO_ 173 



means of a simple screen. This fact lias been made very ap- 

 parent by Prof. J. E. Smith, who, by the nse of a simple 

 " oblique diaphragm," as it is called, has secured results which 

 were previously supposed to demand much more complicated 

 and expensive arrangements. This oblique diaphragm, or 

 screen, consists of a plate of very thin metal secured to the 

 under surface of the stage. The stage then forms the upper 

 surface of a >, and the metal plate the lower one. The angle 

 may be adjusted at will by simply bending the plate, and it is 

 evident that all rays from below will be entirely excluded. 

 A piece of the thin iron known as ferrotype ptate is the cheap- 

 est and best material for this purpose. Its surface should be 

 dead black. 



The same result is obtained by the use of a conical dia- 

 phragm fitted to the sub-stage. 



When such objects as difficult tests are viewed by oblique 

 light, it will be found that there is a certain angle of illumina- 

 tion at which the objective will generally perform best, and 

 this must be found by careful experiment. The higher the 

 angle of aperture of the objective, the greater may be the angle 

 at which the object is illuminated, but it will be found that 

 many objectives fail to work up to the full angle claimed for 

 them by their makers. 



Such are the general principles to which the student must 

 pay attention in regard to the matter of illumination, but in ad- 

 dition there are numerous minor details, a knowledge and appre- 

 ciation of which can only be acquired by practice. The finer 

 objectives are so sensitive to the slightest changes, that the 

 least movement of the mirror or lamp influences the result in 

 a very marked degree. 



The other important point to be attended to is the cover- 

 correction. This will be found to demand great patience and 

 attention. The older authorities give fixed rules for regulating 

 the cover-adjustment, but as it unfortunately happens that this 

 adjustment varies not only with the thickness of the cover 

 glass, but with the depth to which the object is sunk in the 

 mounting medium (and this is not always the same), and the 

 angle of illumination, it will be seen that each object requires 

 special attention in regard to these points. In addition to this, 



