RESOLVING POWER. — FLATNESS OF FIELD. 173 



Aperture, that is, to the obliquity of the rays -which it can 

 receive from the several points of the surface of the object. This 

 is not so much the case where the markings depend upon the 

 interposition of opaque and semi-opaque pai'ticles in the midst of a 

 transparent substance, so that the lights and shadows of the image 

 represent the absolute degrees of greater or less transparence in 

 its several parts ; as it is where, the whole substance being equally 

 transparent, the markings are due to the refracting influence which 

 inequalities of the surface exert upon the course of the rays that 

 pass through it (§ 27). It may be readily perceived, on a little 

 reflection, that the information given about such inequalities by 

 rays of light transmitted axially through the object must be very 

 inferior to that which can be gained from rays of light transmitted 

 obliquely ; and thus it happens that, as already explained (§§ 119, 

 120), many such markings are seen by Oblique illumination, which 

 could not be seen under the same Object-glass by light transmitted 

 more nearly in the axis of the Microscope. When an object, how- 

 ever, is seen by transmitted light, no degree of obliquity in the 

 illuminating rays can be useful, which exceeds that at which the 

 Object-glass can receive them ; but the illumination of objects 

 which are seen by radiated light (§ 86) depends upon these very 

 rays ; and thus it is that the ' black-ground ' illumination by the 

 Paraboloid or by any other effective contrivance (§ 85) will often 

 bring surface -markings into view, which cannot be seen by trans- 

 mitted light. An Object-glass of very wide aperture, however, 

 will receive, even with axial illumination, so many rays of great 

 obliquity, that the same kind of effect will be produced as by 

 oblique illumination with an Objective of smaller aperture ; but 

 when oblique illumination is used with the former, a greater 

 resolving power is obtained than the latter can afford. In com- 

 paring 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 instrument will 'bring-out' tests which 

 another does not resolve with Object-glasses of much greater 

 capability, 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 of the same focal length, 

 one perfectly corrected up to a moderate angle of aperture, the 

 other with a wider aperture but less perfectly 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 practically 



