DEFINING AND PENETRATING POWERS. 187 



objects, must be very superior, for everything else, to a glass which 

 will not show these ; but this is known to every practical Micro- 

 scopist to be a great mistake, the very qualities which enable it 

 to resolve the more difficult "tests" being incompatible with 

 those which make it most useful in all the ordinary purposes of 

 scientific investigation. Four distinct attributes have to be 

 specially considered, in judging of the character of an object- 

 glass, viz. : (1) its defining power, or power of giving a clear and 

 distinct image of all well-marked features of an object, especially 

 of its boundaries ; (2) its penetrating power, or power of enabling 

 the observer to look into the structure of objects; 1 (3) its resolving 

 power, by which it enables closely-approximated marking to be 

 distinguished ; and (4) the flatness of the field which it gives. 



I. The "Defining power"of an objective mainly depends upon 

 the perfection of its correctness, both for Spherical and for Chro- 

 matic aberration ( 9, 15) ; and it is an attribute essential to the 

 satisfactory performance of any objective, whatever be its other 

 qualities. Good definition may be more easily obtained with 

 lenses of small or moderate, than with lenses of large angular 

 aperture ; and in the aim to extend the aperture, the perfection 

 of the definition is not unfrequently impaired. An experienced 

 Microscopist will judge of the defining power of a lens by the 

 quality of the image which it gives of almost any object with 

 which he may be familiar; but there are certain "tests," to be 

 presently described, which are particularly appropriate for the 

 determination of it. Any imperfection in defining power is ex- 

 aggerated, as already pointed out ( 22, 80), by the use of " deep" 

 eye-pieces ; so that, in determining the value of an objective, it 

 is by no means sufficient to estimate its performance under a low 

 eye-piece, an image which appears tolerably clear when mode- 

 rately magnified, being often found exceedingly deficient in 

 sharpness when more highly amplified. The use of the draw- 

 tube ( 43) affords an additional means of testing the defining 

 power ; but this cannot be fairly had recourse to, unless an altera- 

 tion be made in the adjustment for the thickness of the glass 

 that covers the object ( 84), in proportion to the lengthening of 

 the body, and the nearer approximation of the object to the ob- 

 jective which this involves. 



II. The "Penetrating power" of an object-glass (good defini- 

 tion being of course presupposed) mainly depends upon the 



1 The Author is aware that he is here employing the term " Penetration" in a sense 

 very different from that which it was intended to convey by Dr. Goring, who first ap- 

 plied it to designate a certain quality of Microscopic objectives. But he considers that 

 what was termed "penetration" by Dr. Goring may be far more appropriately desig-' 

 nated as resolving power this term having been long in use to express the parallel attri- 

 bute of Telescopes, as regards the separation of the diffused luminosity of Nebulae into 

 distinct points of light. The term Penetration, having been thus set free, may well be 

 applied (as above) in what seems its natural meaning; and the Author (who has long 

 been in the habit of employing it in this sense) may refer to the Report of the Jury of 

 the " Great Exhibition" of 1851, as giving an authoritative sanction to the above use of it 



