PENETRATING POWER, OR EOCAL DEPTH. 171 



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 maybe familiar; but there are certain 'tests,' to be 

 presently described, which are particularly appropriate for the de- 

 termination of it. Any imperfection in Defining power is exag- 

 gerated, as already pointed out (§§ 22, 110), 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 moderately 

 magnified being often found exceedingly deficient in sharpness 

 when more highly amplified. The use of the Draw- Tube (§ 63) 

 affords an additional means of testing the Defining power ; but 

 this cannot be fairly had recourse to, unless an alteration be made 

 in the adjustment for the thickness of the glass that covers the 

 object (§ 113), in proportion to the nearer approximation of the 

 object to the Objective which the lengthening of the body involves. 

 ii. The Penetrating power or Focal Depth of an Object-glass 

 (good definition being of course presupposed) mainly depends upon 

 the degree of distinctness with which parts of the object that are 

 a little out of focus can be discerned ; and this will be found to 

 vary greatly in different Objectives, being, within certain limits, 

 in an inverse proportion to the extent of the Angle of Aperture* 

 (§ 10), as can be easily proved on Optical principles, t Hence an 

 Objective of comparatively limited angular aperture may enable 

 the observer to gain a view of the whole of an object, the several 

 parts of whose structure lie at different distances from it, suffi- 

 ciently good to afford an adequate idea of the relation of those 

 parts to each other ; whilst if the same object be looked at with 



* As the young Microscopist may be perplexed by the fact that an 

 Objective having a large actual aperture may have but a small angular 

 aperture, and that the lenses of largest angular aperture may be those 

 of the smallest actual aperture, it may be well to recall his attention to 

 Fig. 10; from which he will see that the Angle of aperture a b c depends 

 on the actual aperture of the Objective, and the distance of the object 

 (when in focus) from its front lens, conjointly. Hence two Objectives 

 may have the same actual aperture, and yet one may have a much 

 larger angular aperture than the other, because the focal distance of the 

 object is less. On the other hand, two lenses may have the same 

 angular aperture, yet the actual aperture of one shall be much greater 

 than that of the other, the focal distance of the object being greater. 

 And thus, as a general rule, Objectives of low power or long focal dis- 

 tance have the largest actual apertures ; whilst those of high power or 

 short focus have the largest angular apertures. If the focal distance be 

 constant, the angular aperture will increase or diminish with the actual 

 apei-ture ; whilst, if the actual aperture be constant, the angular aper- 

 ture will increase with the shortening of the focal distance, and will 

 decrease with its elongation. 



t Thus the Portrait-lens of a Photographic Camera having a large 

 angle of aperture, is quite unsuitable for Landscape purposes ; and the 

 greater the range of distances it is desired to obtain in a photographic 

 picture (as, for example, in taking the interior of a long Sculpture 

 Gallery, or a Landscape with near fore-ground and remote back- 

 ground), the more must the aperture of the lens be reduced by 'stops.' 



