456 ON APERTUHE AND DEFINITION OF MICROSCOPE OBJECT GLASS. 



are not large so that their points do not suffer much dispersion — 

 yet, still the combined effect will be marred in proportion as the 

 object is not uniformly free from spherical aberration over the whole 

 area of apertare. 



Again, objectives may be constructed with wide aperture, and 

 the focussing of the most inclined pencils be effected with sufficient 

 accuracy, whilst the central zones are left very deficient and incapable 

 of correction. The " definition " of such objectives will be poor 

 even for the particular class of objects for which wide aperture is 

 needed, whilst their definition for ordinary objects will be worse than 

 in good glasses of narrower aperture. This case is far from rare. 



Thus then mere aperture is not decisive of the value of an 

 objective, and the struggle for a few degrees of extra aperture may 

 often prove absolutely injurious to definition. It may, indeed, be 

 possible to correct the defects and surmount all difficulties attendant 

 on the use of largest possible aperture even in dry objectives, but 

 to estimate objectives by mere comparison of their respective 

 apertures is as useless as to deny, on the same grounds, the proved 

 excellence of objectives of moderate aperture (100°) which can and 

 do combine resolving and defining capacity commensurate with high 

 magnifying power. Lastly, it appears that the highest resolving 

 power associated with largest aperture is of value only for a special 

 class of objects, namely, such as consist of finely ruled artificial 

 lines, networks with transparent interspaces, or such as contain in 

 their substance, otherwise homogeneous and transparent, minute 

 particles of differing refractive power which cause diffraction. But 

 all such objects, when employed as tests for resolving power, fail to 

 exhibit the general and more useful qualities of objectives con- 

 structed to define as accurately as may be in one picture ^with each 

 fresh focal adjustment) elements of variously irregular figure and 

 light-absorbing power situate in focal planes of varying depths 

 where a resolving capacity of go otto" ^^^^ would not penetrate. 

 Even in the case of particles held in suspension in a more or less 

 transparent fluid, the best combination of resolving and defining 

 capacity would avail far more than extreme resolving power which 

 might suggest some indefinable reality. 



