RESOLVING POWER FLATNESS OF FIELD. 189 



the condenser, 56), which could not be seen, under the same 

 object-glass, by light transmitted more nearly in the axis of the 

 microscope. When an object, however, is seen by transmitted 

 light, no degree of obliquity in the illuminating rays can be use- 

 ful, which exceeds that at which the object-glass can receive 

 them: but the illumination of objects which are seen by radiated 

 light ( 62), depends upon these very rays ; and thus it is that 

 the " black-ground" illumination by the paraboloid or by any 

 other effective contrivance ( 61), will often bring surface-mark- 

 ings into view, which cannot be seen by transmitted light. An 

 object-glass of very wide aperture, however, will receive, even 

 with ordinary illumination, so many rays of great obliquity, that 

 the same kind of effect will be produced, as by oblique illumina- 

 tion with an objective of smaller aperture ; but when, with such 

 an objective, oblique illumination is used, a greater resolving 

 power is obtained, than any combination of smaller angular aper- 

 ture can possess. In comparing the resolving power of different 

 object-glasses, it is obviously essential to a correct judgment, 

 that the illumination should be the same ; for it will often happen 

 that an observer who knows the "points" of his own instru- 

 ment, will "bring out" tests, which another, with object-glasses 

 of much greater capability, does not resolve, simply for want of 

 proper management. Moreover, it must be borne in mind that 

 great resolving power may exist, even though the definition may 

 be far from exact ; since the former depends more upon angle 

 of aperture, than upon the perfection of the corrections : and yet 

 there cannot be the slightest question, that, of two objectives ot 

 the same focal length, one perfectly corrected up to a moderate 

 angle of aperture, the other with the wider aperture but less per- 

 fectly corrected, the former will be the one most suitable to the 

 general purposes of the Microscopist. 



IV. The " Flatness of the field" afforded by the object-glass, 

 is a condition of great importance to the advantageous use of 

 the Microscope ; since the real extent of the field of view prac- 

 tically depends upon it. Many objectives are so constructed, 

 that, even with a perfectly flat object, the foci of the central and 

 of the peripheral parts of the field are so different, that when 

 the adjustment is made for one, the other is entirely indistinct. 

 Hence, when the central portion is being looked at, no more in- 

 formation is gained respecting the peripheral, than if it had been 

 altogether "stopped out." With a really good object-glass, not 

 only should the image be distinct even to the margin of the 

 field, but the marginal portion should be as free from chromatic 

 fringes or from indistinctness of outline, as the central portion. 

 In many microscopes of inferior construction, the imperfection 

 of the objectives in this respect, is masked by the contraction of 

 the aperture of the diaphragm in the eye-piece ( 21), which 

 limits the dimensions of the field ; and the performance of one 

 objective within this limit may scarcely be distinguishable from 



