OJSr APEKTTJEE AND DEFINITION OF MICEOSCOPE OBJECT GLASS. 443 



into small divergent diffraction pencils, which, entering a wide 

 angled objective, form '-positive" images (with, perhaps, coloured 

 fringes) of the particles or lines which caused the diffraction. These 

 images are '' positive " because they repeat the self-luminous 

 character which marks their peculiar mode of origin. But, though 

 delineated in accordance with dioptric law, they differ from the 

 ''negative" images formed by non-diffracted rays (that is to say, 

 by the ordinary pencils of light) in one important respect, namely, 

 that, owing to the greater dispersion of such diffracted pencils 

 before reaching the objective, a large apertnre only can admit them, 

 and the diffracted pencils which form these positive images have, 

 therefore, a greater inclination to the axis. 



If now "definition" be interpreted to mean accurate geometric 

 delineation of a microscope image, it is manifest that the source of 

 "definition" is not "angular aperture" of an objective, but 

 accurate fociissmg function of a lens or system of lenses. And, so 

 far as concerns the diffraction images admitted in virtue of large 

 aperture, they may be a cause of deterioration of the general effect, 

 unless the spherical aberration of rays having an extreme inclination 

 to the axis of the instrument, be so corrected that the focussing 

 function shall bring the diffraction images into perfect correspondence 

 with the negative images formed at the same moment. And again, as 

 respects the diffraction effect caused by the minuteness of optical 

 aperture of the whole system of lenses of the compound microscope 

 (including the ocular), through which the final image is viewed by 

 the eye, the good effect of large " aperture " of objective is also 

 contingent upon the perfection of its focussing function, that is to 

 say, depends upon good definition instead of heing the cause of it. 



The view here briefly expressed accords strictly with the observa- 

 tions of Professor Helmholtz, and the researches of Professor Abbe, 

 and I propose, as the subject is not without interest at the present 

 moment, to discuss it a little more fully in the following pages. 



In the first place Helmholtz has himself, in the first j)art of his 

 essay, indicated the true nature of definition by showing its depen- 

 dence on the aperture, not of the objective, but of the illuminating 



