MANAGEMENT OF THE MICROSCOPE. 163 



clear when moderately magnified, being often found exceedingly deficient 

 in sharpness when more highly amplified. The use of the Draw-tube 

 ( 83) affords an additional means of testing the Defining power; but 

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

 adjustment for the thickness of the glass that covers the object ( 139), 

 in proportion to the nearer approximation of the object to the Objective 

 which the lengthening of the body involves. 



in. The Penetrating power or ' focal depth ' of an Object-glass may 

 be defined as consisting in the vertical range through which the parts of 

 an object not precisely in the focal plane may be seen with sufficient 

 distinctness to enable their relations with what does lie precisely in that 

 plane to be clearly traced out; just as we could do by ordinary vision, if 

 the object were itself enlarged to the size of its Microscopic image. Now 

 this is a quality which is very differently valued by different observers, 

 according to the nature of the work on which they may be severally 

 engaged. The Histologist who is scrutinizing the elementary compo- 

 nents of a tissue that is spread out in the thinnest possible film between 

 two plane surfaces of glass, considers ' penetration ' rather an evidence of 

 imperfection in his Objective, which (he affirms) cannot show him 

 anything save what is exactly in the focal plane, without a sacrifice of its 

 highest attainable capacity for doing the latter. On the other hand, the 

 Anatomist who is studying the general organization of some minute 

 Plant or Animal, or the structure of individual organs in a larger one, 

 finds a certain amount of f penetration ' essential to his recognition of 

 the relations between the several parts of the object which are suc- 

 cessively brought into distinct views by alterations of the focal adjust- 

 ment. And the Physiologist who is watching the actions that are going 

 on in a living Organism or in some component part of it (as, for exam- 

 ple, the internal movements of an Amoeba, or the cyclosis in a leaf -cell of 

 Vallisneria) could form no satisfactory conception of such phenomena, 

 if, instead of passing gradationally (as an Objective of good ' penetration ' 

 allows him to do) from one focal plane to another, he can only get a 

 series of ' dissolving views ' with an interval of ' chaos ' between each, as he 

 does when working with an Objective whose ' penetration ' has been sacri- 

 ficed to Angular aperture. For the study of opaque objects which 

 present such inequalities of surface as to render it impossible to appre- 

 hend their true forms unless much more can be seen than is precisely in 

 focus at once, good i penetrating' power is obviously essential; and this 

 is indispensable to the advantageous use of the Stereoscopic Binocular, 

 which grossly exaggerates the effect of projection, when objects are 

 viewed under Objectives of too wide an angle ( 39). No definite rule 

 can be laid down as to the relation which the 'focal depth' of an 

 Objective bears to its 'working distance' and its 'angular aperture;' 

 because much depends upon the mode of their construction. But it may 

 be stated generally that Objectives of longest working distance have the 

 greatest 'penetration;' whilst the widening of the Angular aperture 

 diminishes penetration at a rapidly increasing rate. 1 



1 The Author is informed by Prof. Abbe, that, theoretically the plan of con- 

 struction remaining the same the ' penetration ' of an Objective decreases, as the 

 square of the Angular aperture increases. It is perfectly well-known to Photo- 

 graphers, that a good picture of the interior of a long Sculpture-gallery, showing 

 both the near and the distant parts with tolerable distinctness, can only be 

 obtained by a lens of very narrow angle. The singular assertion lately made by 

 Dr. Blackham ( On Angular Aperture of Objectives," New York, 1880), that 



