The Relation of Aperture and Power. Bij Prof. E. Able. 309 



working microscopist. Bad lenses will certainly not meet the 

 demand for low and medium powers affording the utmost possible 

 economy of time and labour in scientific work. This can be done 

 only by systems in which all advantages attendant upon the lower 

 apertures are fully realized by constructions specially aiming at 

 the best which can be obtained under the actual conditions of the 

 case. 



The ^progressive increase of aperture in the higher powers, for- 

 merly within the capabilities of the dry system, and at a later period 

 by the development of the immersion method, is, without any reason- 

 able doubt, the most important feature of the modern advance of 

 microscopical optics. It has rendered possible the successful ex- 

 tension of microscopical research to minuter and minuter objects, 

 which otherwise would have been impossible by the inefiectiveness 

 of all increase of amplification beyond certain low figures. The 

 appreciation of that progress and the recognition of its true basis 

 has led to a tendency to increase more and more the aperture of 

 every kind of objectives. The fact has been disregarded that it is 

 an entirely difi'erent thing whether the object is to promote 

 the performance of the Microscope in the whole at the limits of its 

 power, or to promote its performance for aims beyond these limits. 

 The opinion has thus arisen that what is a benefit for one kind of 

 lenses must also be a benefit for every other kind. Objectives of 

 low and medium powers (1-inch to ^-inch) of 15° to 60° are pro- 

 claimed at this time by many microscopists as old-fashioned and 

 worthless things; 45° to 100°, or even 60° to 140°, are wanted 

 for the same powers. Now as from a purely technical point of 

 view, it is an accomplishment when the delineating power of an 

 objective cannot be exhausted even with the deepest eye-pieces, 

 opticians (notwithstanding the total bootlessness of such a super- 

 abundance) of course take pleasure in making such "superior" 

 lenses, and the natural consequence is that the lower apertures 

 required for useful scientific research are likely to be esteemed as 

 second-rate work, no longer worthy of high technical art. 



This opinion is a fatal mistake, and its practical efiect, if not 

 counteracted, will be a decided retrogradation of microscopical 

 optics. Nobody, of course, can have the least objection to the 

 construction of lenses of any descripition whatever for the personal 

 pleasure of this or that microscopist. Strong opposition should, 

 however, be made against all tendencies of captivating microscopical 

 optics, in favour of such predilections, at the cost of the general 

 usefulness of the instrument. 



Scientific work ivith the Microscope will always require not 

 only high-power objectives of the widest attainable apertures, 

 but also carefully finished loiver pavers of small and very 

 moderate apertures. 



