THE OPTICAL EQUIPMENT 25 



it is best, if such an arrangement is possible, to immerse the 

 object in fluid : by this means the greatest possible amount of 

 penetration is obtained. 



Another quality of an objective which, particularly in low- 

 power lenses, is of importance, is what is known as flatness of 

 field. With no objective is the whole field of view in proper focus 

 at one time at least, when a moderate-power eye-piece is used. 

 With some oculars this does not appear to be true, but it is only 

 due to the diaphragm in the ocular restricting the size of the 

 visible field. It would appear that the increase of numerical 

 aperture has tended to decrease flatness of field somewhat, 

 as a small field is characteristic of objectives of large aperture, 

 especially of the high-power ones. It is at times convenient 

 to have a lens that covers a large field, even if the definition 

 is not so absolutely critical as might otherwise be desirable. 

 But, in general, it will be found that lenses of otherwise good 

 quality have sufficient covering-power for any ordinary require- 

 ment. The writer is of opinion that flatness of field should not 

 be regarded as a point of first importance ; and a lens with this 

 quality should only be selected when all other requirements have 

 been fulfilled. It is a quality easy to determine, since with 

 any flat object it can at once be seen where definition begins to 

 fall off, and what proportion of the field is actually in critical 

 focus at one time. 



It may incidentally be remarked that with a good objective, 

 although the whole field of view may not be in focus at one time, 

 any alteration of the fine adjustment should result in successive 

 zones of the field coming into focus and the quality of the 

 image should be practically as good at the margin of the 

 field as it is at its centre. 



On the numerical aperture of an objective depends its 

 power of making visible and separating fine details in an object, 

 and it is dependent upon the size or angle of the cone of light 

 collected by the objective from each point in the object. It 

 must not be confused with angular aperture, or the method of 

 indicating the aperture of photographic lenses, as these are 

 dependent on the total angle of the field of view. Numerical 

 aperture is purely a function of the objective, the ocular only 

 amplifying the image, and adding nothing to the resolving 

 power of the optical combination, 



