ON APERTURE AND DEFINITION OF MICROSCOPE OBJECT GLASS. 451 



A better understanding of the part played by aperture in 

 defining (as disfcinguisbed from resolving) microscopic objects may 

 be gathered from the essays of Helmholtz and Abbe, translations 

 of -which have been given in the preceding pages, than from any 

 misapplication of a formula intended only as a mathematical ex- 

 pression of the possible limits of resolution. The readers of these 

 essays will have seen that to each of these terms, long naturalised 

 in micrography, and supposed to convey a separate and distinct 

 meaning, a specific function answering to specific physical 

 phenomena may be truly assigned. In this, as in other scientific 

 enquiries, all depends on a clear and accurate definition of the terms 

 employed, especially when such terms are intended to convey some 

 explanation of complex phenomena and combined effects. For to 

 such combined effect, the reference made by Mr. Sorby in the 

 passage before quoted from his presidential address, was, doubtless, 

 intended to apply. But it seems a pity to endanger the most 

 instructive results of modern research — the fruit of much profound 

 labour — by any loose application of a single term to cover all the 

 physical phenomena concerned in the formation of a well defined 

 optical image. The foundation of the whole series of phenomena 

 which have to be analysed in a theory of the microscope is that 

 dioptric law governing the focassing and magnifying functions, 

 which obtains equally in every objective whatever be its angular 

 aperture ; and ** definition" depends alike in small or large angled 

 objectives upon correction of spherical aberration. Increase of 

 angle beyond llO*' only throws difficulties in the way of the 

 optician as regards the maintenance of even moderately perfect 

 definition. 



For the oblique incidence of the outermost pencils^the prime 

 cause of spherical aberration — is the main characteristic of large 

 aperture. And the possible gain of defining pencils of large 

 divergence angle, as well as the possible addition of new details 

 admitted with diffraction pencils (from the object) through the wide 

 angled aperture, will be realised only in proportion to the dioptric 

 perfection of focussing function. In respect to " resolving " 



