164 THE MICROSCOPE AND ITS REVELATIONS. 



iv. The ' Kesolving power ' by which very minute and closely approx- 

 imated markings whether lines, striae, dots, or apertures are separ- 

 ately discerned, has now been clearly shown to depend upon Angular 

 aperture ( 157); and this, not so much as formerly supposed on account 

 of the greater obliquity of the rays which large-angled Objectives will 

 admit, as because of their capacity to receive and recombine the ' dif- 

 fraction-spectra ' that lie without the range of Objectives of more lim- 

 ited angle. In comparing the 'resolving' powers of different Objec- 

 tives, it must be borne in mind that the advantages of wide aperture 

 will be lost, if the obliquity of the illumination does not correspond with 

 that of the most divergent rays which enter the Objective to take part in 

 the formation of the image. But when the question is not of the reso- 

 lution of surface-markings (such as those of Diatom-valves), but of the 

 determination of internal structure (as, for example, in the study of the 

 process of division in cell-nuclei), axial illumination is decidedly to be 

 preferred, as being attended with less liability than oblique to produce 

 deceptive appearances. It appears from the theoretical researches of 

 Prof. Abbe, that the maximum attainable resolving power with an An- 

 gular aperture of 180 should separate 118,000 lines to the inch; and 

 this agrees well with what has been actually accomplished ( 160). But 

 the loss of ' resolving ' power consequent upon the contraction of the 

 aperture from 180 to 128-J is only 10 per cent.; while a further reduc- 

 tion to 106 J only lowers the number of separable lines to 94,400 per 

 inch. 



V. The ' Flatness of the field ' afforded by the Object-glass is a con- 

 dition of great importance to the advantageous use of the Microscope, 

 since the real extent of the field of view practically depends upon it. 

 Many Objectives are so constructed, that, even with a perfect flat ob- 

 ject, the foci of the central and of the peripheral parts of the field are so 

 different, that when the adjustment is made for one, the other is ex- 

 tremely indistinct. Hence, when the central portion is being looked at,, 

 no more information is gained respecting the peripheral, than if it had 

 been altogether stopped out. With a really good Object-glass, not only 

 should the image be distinct even to the margin of the field, but the 

 marginal portion should be as free from color as the central. In many 

 microscopes of inferior construction, the imperfection of the Objectives 

 in this respect is masked by the contraction of the aperture of the dia- 

 phragm in the Eye-piece ( 27), which limits the dimensions of the 

 field; and the performance of one Objective within this limit may scarcely 

 be distinguishable from that of another, although, if the two were com- 

 pared under an Eye-piece of larger aperture, their difference of excellence 

 would be at once made apparent by the perfect correctness of one to the 

 margin of the field, and by the entire failure of the other in every part 

 save its centre. In estimating the relative merits of two lenses, there- 

 fore, as regards this condition, the comparison should be made under 

 an Eye-piece giving a large field. 



vi. The most perfect objective for general purposes, is obviously that 

 which combines all the preceding attributes in the degree in which they 

 are mutually compatible. But it seems to be now clear that the highest 

 perfection of the two primary qualities, ' defining' power and ' resolving 



' depth of focus ' has no relation to Aperture, but depends on residual " (i. e., 

 uncorrected) Spherical Aberration, and that " the less the lens has of it, the bet- 

 ter the lens," does not require serious refutation. 



