22 PKACTICAL PHOTO-MICEOGEAPHY 



sharpness in depth did exist. This apparent depth of focus 

 may be obtained by reducing the aperture of the sub-stage 

 condenser, the reduction being effected by closing the sub-stage 

 iris-diaphragm. A narrower cone of rays, therefore, enters the 

 objective, causing it to work at a smaller numerical aperture. 

 This is a very harmful proceeding if carried to excess, and the 

 novice cannot be warned too strongly that such a method is not 

 advisable for the purpose of obtaining penetration, except 

 under very special conditions. For some classes of work lenses 

 of small aperture are still used particularly where it is only 

 desired to obtain a general idea of the appearance of an object 

 of considerable thickness ; but for critical work, whether visual 

 or photographic, lenses of the highest obtainable aperture are to 

 be preferred. Since all the efforts of opticians have in recent 

 years been directed to increasing numerical aperture, this 

 quality of penetration has been gradually reduced, until now 

 it might be regarded as a defect rather than otherwise. In 

 visual work some accommodation usually results from the 

 continual raising and lowering of the objective by the fine 

 adjustment, so that a concrete notion of the appearance of 

 an object in depth is obtained ; but in photography this is 

 not so. 



Although it has been pointed out that the quality of pene- 

 tration is in general not desirable in microscope objectives, it 

 does sometimes happen in actual practice that the delineation of 

 structure in depth, although it may not result in a truly critical 

 representation, is very useful. The method of obtaining this 

 penetration by reducing the aperture of the sub-stage condenser 

 is open to serious objections, and a very much more satisfactory 

 way is to reduce the aperture of the objective itself. With low- 

 power lenses this may be effected by having immediately behind 

 the objective, in the position where it screws into the micro- 

 scope tube, an iris-diaphragm which may be contracted at will. 

 With high-power lenses, however, this is not a practical method, 

 as it is not possible to get a diaphragm sufficiently close to the 

 posterior combination of the objective to stop it down effectively. 

 Both Messrs. Leitz and Zeiss now construct what are in fact 

 small metal funnels that drop into the back of the objective 

 and have an opening which is nearly in contact with the back 

 lens, These contrivances are made so that a definite reduction 



