The Belatmi of AjJerticre and Power. Bij Prof. E. Able. 305 



by means of successive optical sections through successive focus- 

 sing of different planes. For the latter method of observation the 

 loss of penetration with increasing power and aperture is no draw- 

 back, but rather an advantage, because it enhances the distinct 

 separation of the sectional images at successive foci. A disregard 

 of these natural restrictions in the use of wide apertures is 

 obviously the origin of the opinion that aperture jyer se is antago- 

 nistic to good definition. It is quite true that there are many 

 even very delicate objects which are much better seen under a 

 given amplification with a system of very moderate than with one 

 of very wide aperture, the former giving a clear view of the 

 whole structure, the latter showing perhaps some distinct points, 

 but as a whole veiled in haze. Provided, of course, that we have 

 well-corrected objectives, the fault here is not on the part of the 

 lens, but on the side of the object, which requires for proper 

 recognition a greater range of depth than is reconcilable with a 

 wide aperture. The theoretical suggestion which has been brought 

 forward in support of the notion tliat different parts of the clear 

 area of an objective produce dissimilar images, and that therefore 

 the resultant image must show increasing confusion with increasing 

 aperture, cannot apply to the delineation of a plane object. In a 

 well-corrected objective the partial pictures received through the 

 various parts of the aperture-area are always strictly similar so far 

 as one plane of the object is concerned. The confusion suggested 

 is nothing else but confusion of the images of different depths — 

 lack of penetration, but not lack of " definition " in any reasonable 

 sense of that term. Provided the objectives are properly corrected 

 and the objects are fit for the delineation of an image, undisturbed 

 by interfering confused images from other planes, the " defining 

 power" of an objective is always greater with greater aperture for 

 every kind of objects, inasmuch as under all circumstances the wider 

 aperture admits of the utilization of higher amplifications than 

 can be obtained without perceptible loss of sharpness (with the 

 same objects) by lower apertures. 



There is therefore no drawback in principle to the use of a 

 large aperture when the objects are suitable. But the considera- 

 tions above lead to the conclusion ; — 



Wide apertures {together with high 'powers) for those 

 preparations only which do not require perceptible depth of 

 vision, i.e. for exceedingly fiat or thin objects, and for trans- 

 parent objects which can be studied by optical sections. 

 Moderate and low apertures ivhen a wide range of pene- 

 tration cannot he dispensed with. 

 4. There is still another point of view, and one of special 

 practical importance, which shows the positive damage connected 

 with the use of unnecessarily wide apertures. The increase of 



