188 MANAGEMENT OF THE MICROSCOPE. 



degree of distinctness with which parts of the object that are a 

 little out of focus can be discerned ; and' this will be found to vary 

 greatly in different objectives, being, within certain limits, in an 

 inverse proportion to the extent of the angle of aperture. This 

 is very easily understood on optical principles. The central rays 

 of any pencil undergo the least refraction or change in their 

 course ; the peripheral rays, the most . The greater the change, 

 the greater is the difference between the amounts of refraction 

 respectively undergone by rays coming off from points at slightly 

 different distances ; and the greater, when the focal adjustment 

 is correct for one of these points, will be the indistinctness of the 

 image of the other. Hence an objective of comparatively limited 

 aperture may enable the observer to gain a view of the whole of 

 an object, the several parts of whose structure lie at different dis- 

 tances from it, sufficiently good to afford an adequate idea of the 

 relation of those parts to each other; whilst if the same object 

 be looked at with 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 re- 

 lations of the whole cannot be brought into view. The want of 

 this "penetrating power" is a serious drawback in the perfor- 

 mance of many objectives, which are distinguished by the pos- 

 session 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 objectives which are to be em- 

 ployed for the general purposes of scientific investigation, that he 

 cannot consider its deficiency to be compensated by the posses- 

 sion of any degree of the resolving power, whose use is compara- 

 tively limited. 



III. The " Resolving power," by which very minute markings, 

 whether lines, striae, or dots, are discerned and clearly sepa- 

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

 perfect definition being presupposed) to the extent of its angle 

 of aperture, and consequently to the obliquity of the rays which 

 it can receive from the several points of the surface of the ob- 

 ject. This is not so much the case, where the markings depend 

 upon the interposition of opaque or semi-opaque particles in the 

 midst of a transparent substance, so that the lights and shadows 

 of the image represent the absolute degrees of greater or less 

 transparency in its several parts ; as it is where, the whole sub- 

 stance being equally transparent, the markings are due to the 

 refracting influence which inequalities of the surface exert upon 

 the course of the rays that pass through it. It may be readily 

 perceived, on a little reflection, that the information given about 

 such inequalities by rays of light transmitted axially through the 

 object, must be very inferior to that which can be gained from 

 rays of light transmitted obliquely ; and thus it happens that, 

 as already explained, many such markings are seen by oblique 

 illumination (as, for instance, by the use of the central stop in 



