RELATIVE POWER OF DISCRIMINATION. 145 



As regards the chromatic aberration of the eye which reaches 

 a very marked amount, occasioning principally the phenomena 

 of irradiation it is conceivable, that it could be eliminated by 

 a rather over-corrected Microscope, and thus an increase of 

 discriminating power could be obtained. This explanation is 

 certainly applicable to Beneche's No. 4 objective, which is never 

 capable of compensating in the whole middle part of the field 

 of view the opposite chromatic aberration of the eye, although 

 distinctly over-corrected. We will discuss later on the testing 

 of such phenomena, noting the simple fact here, in order to 

 establish without doubt the correctness of the explanation for 

 at least one definite case. 



The diagram shows, secondly, that the relative optical power of 

 Microscopes decreases considerably in proportion to the higher 

 and highest amplifications, since the corresponding curves ap- 

 proach the abscissa-axis rather rapidly towards the right, in the 

 most unfavourable cases sinking to even less than half their 

 height. We might hence conclude, that practical constructions are 

 still far from the limit of the attainable ; and so far as regards 

 the perfection of the dioptric image merely, this conclusion would 

 be wholly justified. As, however, at the limit of the discri- 

 minating power the interference image is exclusively operative, 

 the minuteness of the details depending chiefly upon the extent 

 of the angular aperture, the case is essentially altered. The angle 

 of aperture of the highest objectives, having 180 in air, has 

 already reached the limit which is in general attainable, and 

 the power of discrimination determined by it is raised to about 

 that amount which for physical reasons it is impossible to exceed. 1 

 On the other hand, there is still required a greater uniformity 

 of correction in the different zones of the objective, as well as 

 more harmony in the combined action of the various factors 

 which determine the optical power of a system, at all times a 

 very desirable aim, and by no means yet attained in practice. 



For the medium amplifications, 300 400, the objectives of 

 the various opticians we have mentioned are nearly equal with 

 regard to the power of discrimination, while with the low-power 

 objectives not unimportant differences appear. The explanation 

 of these differences lies not so much in the impossibility of 

 cancelling them, as in the absence of a desire to do so, because 



1 Cf . upon this point our chapter on the Theory of Microscopic Observation. 



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