172 COMPARATIVE VALUE OF OBJECT-GLASSES. 



an Objective of very wide angle of aperture, which only enables 

 what is precisely in focus to be seen at all, each part can only be 

 separately discerned, and the mutual relations of the whole can- 

 not be brought into view. The want of this Focal Depth is a 

 serious drawback in the performance of many Objectives which 

 are distinguished by the possession of other admirable qualities. 

 The possession of a high measure of it is so essential, in the 

 Author's opinion, to the satisfactory performance of those Objec- 

 tives which are to be employed for the general purposes of 

 Scientific investigation, that he cannot consider its deficiency to be 

 compensated by the possession of any degree of the Resolving 

 power, whose use is comparatively limited. The value of Pene- 

 trating power is especially felt when the Binocular arrangement 

 is employed ; since the assistance which it is able to give in the 

 estimation of the solid forms of objects is in great degree neu- 

 tralized by the employment of Objectives of such wide angular 

 aperture as not to show any part of the object distinctly save what 

 is precisely in focus ; whilst, in addition, those forms are untruly 

 represented through the exaggeration of projection occasioned by 

 the too great dissimilarity of the pictures received through the 

 two halves of the Objective (§ 30). And the Author has found 

 that all who have made much use of this instrument are now 

 come to an agreement as to the superior value of Objectives of 

 a moderate, or even a comparatively small, Angle of Aperture 

 for ordinary working purposes ; the special utility of the very 

 wide apertures being limited to particular classes of objects. 



in. The 'Resolving power,' by which very minute markings — 

 whether lines, stria?, or dots — are discerned and clearly separated 

 from each other, may be said to stand in direct relation (a perfect 

 definition* being presupposed) to the extent of its Angle of 



* Of the various modes which have been proposed for measuring the 

 Angle of Aperture of Microscopic Object-glasses, the following is one of 

 the simplest and most convenient : — The Microscope is to be placed 

 perpendicularly on a table covered witb dark cloth, and is to be used 

 after the manner of a diminishing Telescope, the ordinary Eye-piece 

 being removed, and a common pocket or watchmaker's hand-glass of 

 two or three inches focal length being held at such a distance from the 

 Objective as to give a distinct image of objects lying on the surface of 

 the table. A strip of white cardboard or paper is then to be laid on 

 either side of the centre of the field of view, and to be gradually 

 moved outwards until its edge is just vanishing ; then if lines be 

 drawn from the centre of the front glass of the Objective to the inner 

 edges of these strips, the angle included between them will be that of 

 the aperture of the Object-glass ; and it may be either measured by an 

 ordinary graduated scale or protractor, so held that its straight edge 

 shall be parallel to the table, whilst the central point of that edge shall 

 coincide with the centre of the front lens of the Objective ; or it may 

 be calculated by dividing half the horizontal distance between the card- 

 board edges by the vertical distance of the Objective from the table, 

 and finding in a table of Natural Tangents the angle corresponding to 

 the product, which, when doubled, will be the Angle of Aperture. This 

 is the true available angle for the formation of distinct images ; and 

 will be found in many cases considerably less than the angle of admission 

 of diffused light. 



