MANAGEMENT OF THE MICROSCOPE. 161 



and (2) that, as there is a like increase in the number of separate 

 diffraction spectra which can be combined with the dioptric image, the 

 representations of minute structure given by Objectives of widest Angular 

 aperture are more trustworthy than those given by those of narrower. 



158. Relative Qualities of Objectives. In estimating the comparative 

 values of different Objectives, regard must always be had to the purpose 

 for which each is designed; since it is impossible to construct a combina- 

 tion which shall be equally serviceable for every requirement. It is 

 commonly assumed than an Objective which will show certain Test- 

 object s, must be very superior for everything else to a glass which will 

 not resolve these; but this is known to every practised Microscopist to be 

 a complete mistake, the qualities which enable it to resolve some of the 

 more difficult ' tests/ not being by any means identical with those which 

 make it most useful in all the ordinary purposes of Scientific investiga- 

 tion. Five distinct attributes have to be specially considered in judging 

 of the character of an Object-glass, viz. (1) its working-distance, or 

 actual interval between its front lens and the object on which it is 

 focussed; (2) its defining power, or power of giving a clear and distinct 

 image of all well-marked features of an object, especially of its bounda- 

 ries; (3) it penetrating power, or focal depth, by which the observer is 

 enabled to look into the structure of objects; (4) its resolving power, by 

 which it enables closely-approximated markings to be distinguished; and 

 (5) the flatness of the field which it gives. 



i. The ' Working distance ' of an Objective has no fixed relation to 

 its 'focal length;' the latter being estimated by its equality in magnify- 

 ing power with a single lens of given curvature; 1 while the former varies 

 with the mode in which the combination is constructed, and with the 

 angular aperture given to it. Of two Objectives of 1-inch focus and the 

 same angle of aperture (say 25), one may have, in virtue of its construc- 

 tion, a much longer ' working distance ' than the other; and this is not 

 only an advantage in facilitating the side-illumination of opaque objects, 

 but also in admitting (as will presently appear) of greater ' focal depth ' 

 or 'penetration.' But it is especially in the case of high powers that 

 ' working distance ' comes to be of essential importance. The widening 

 of angular aperture which is required to give them their highest degree 

 of 'resolving' power (iv.) necessitates a very close approximation of the 

 front lens to the object; and whilst it is an absolute necessity that the 

 interval should be sufficient for the interposition of a cover of the thin- 

 nest glass, or (if this be inadmissible) of a film of mica, every addition to 

 this interval is a clear gain, not only in convenience of working, but also 

 in regard to the 'penetrating' power (in.) of the Objective. The 

 increase of ' working distance ' obtainable by the use of the Immersion 

 system is by no means the least of its advantages. 



ii. The 'Defining power' of an Objective depends upon the com- 

 pleteness of its corrections for Spherical and Chromatic aberration ( 9- 



the rays, by the medium whether Air, Water, Glycerine, Balsam, or Oil 

 through which they pass in their course from the object into the Objective. (See 

 Appendix.) 



1 Owing to the want of some common standard, Objectives constructed by 

 different Makers of the same nominal focal length, often differ considerably from 

 each other in magnifying power; and the proportional amplification given by the 

 different Objectives of any one Maker's series is often very different from that 

 indicated by their nomenclature. It is therefore greatly to be wished that some 

 uniform standard could be agreed on; such as that of Magnifying power under 

 an Eye-piece of definite focal length, at a fixed distance from the Objective. 

 11 



