ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 317 



case of a dry objective, there seems to be a falling off wben for tbe 

 dry angle of 170° is substituted the balsam-angle of 80°. 



Thus an oil-immersion objective of 100° angular aperture (1-16 

 num. ap.) has in reality a greater resolving power than a dry 

 objective of 170° angular aperture (0"99 num. ap.). But vrhen an 

 object is observed, first dry and then in balsam, its structure is much 

 less visible in the latter case than in the former, whence it is con- 

 cluded that as regards resolving power, at any rate, the greater angle 

 has the greater effect. 



The diminished visibility is, however, conditional, and has 

 nothing to do with the resolving power of the objective, as it exists 

 only in the case of those objects whose refractive indices nearly 

 approach that of the medium in which they are immersed, whereby 

 their minute structure is rendered the less distinct. As Mr. Stephen- 

 son has pointed out,* the image of the balsam-mounted object has 

 become fainter in consequence of this nearer approximation to equality 

 of the diatomaceous silex and the balsam of the mounting ; the mark- 

 ings, whatever they may be, are less pronounced than when in air, the 

 visibility being proportional to the difference between the refractive 

 indices of the object and the mounting medium. 



A simple experiment readily shows whether the reduced visibility 

 of the object is due to the cause we have mentioned or to the reduction 

 of the angle. Substitute for the balsam a mounting substance of 

 greater refractive index. The angle is now still moi-e reduced, and 

 the object should be still less distinct if the view contended for were 

 correct. In fact, it is more distinct, and it is obvious therefore that 

 the reduction of the angle has nothing to do with the matter. 



Eliminating therefore all exceptional circumstances and dealing 

 with resolving power in its essential conditions, it is found both by 

 experiment and by theory that the resolving power of objectives does 

 not vary, as the angular-aperture theory supposes, with the angles. 



It is unnecessary to reproduce here the demonstrations which show 

 that the microscopical image of minute objects is not, as was for so 

 long supposed, a dioptric but a diffraction image, as it is referred to 

 hereafter."!" It is sufficient for the present purpose to note that just as 

 a grating produces a central uncoloured image and lateral spectral 

 images of a candle-flame, so a diatom will produce at the back of the 

 objective central and lateral images of the source of light, more and 

 more of the structure being revealed according as a greater number of 

 the diffraction spectra are taken up by the objective. Thus, as we have 

 seen, a dry objective of 180° will give an emergent beam of limited 

 diameter, and will then admit a given number of diffraction spectra 

 on each side of the central uncoloured image. A water-immersion objec- 

 tive of wide angle will give an emergent beam of greater diameter, 

 by means of which additional diffraction spectra on either side may 

 be brought into the field with increased resolution of the object. An 

 oil-immersion objective giving a still larger emergent beam will, in 

 the same way, bring into the field still more diffraction spectra with 

 further advanced resolution. The divergence of the diffraction spectra 

 * See this Journal, iii. (1880) p. 564. t See IV., infra, p. 347. 



Ser. 2.— Vol. I. Z 



