ON APERTURE AND DEFINITION OF MICROSCOPE OBJECT GLASS. 453 



perfect freedom from spherical aberration must be the first and 

 principal condition of accurate definition, and the existence of 

 dispersion circles in place of pointed pencils constitutes the greatest 

 fault of the image forming process. 



The conditions under which every objective, whether of large or 

 small aperture, must perform its focussing function are contained in 

 the law demonstrated by Professors Helmholtz and Abbe. In order 

 to see in what respects definition is improved, or otherwise, by 

 additional aperture, it is necessary to determine in what respects 

 the objective of larger aperture differs in its mode of action from 

 an objective of small angle, and how this mode of action affects the 

 dioptric conditions of the focussing function. Now it appears, 1, 

 that the objective of wide aperture admits larger divergent pencils 

 than the smaller angled lens ; 2, that, of these larger pencils, such 

 as occupy the peripheral (marginal) zones of the front lens, have a 

 greater inclination to the axis of the instrument than any pencils 

 incident on the lens of narrow aperture ; 3, that these outer zones 

 admit not only larger pencils of light, according to the angle of 

 illuminating cone under the object, but also any rays split up by 

 diffraction due to the action of particles in the object, &c., which 

 fall within reach of the aperture of objective. These three points 

 of difference indicate corresponding differences in the conditions of 

 focussing function in large and small angled objectives respectively 

 In respect to the first (which has a direct bearing upon the theory 

 of diffraction in the microscope) the differences of divergence- 

 angle of the defining pencils of light which enter the narrow or wide 

 aperture of objective, affect, in directly opposite ways, the definition 

 of the image, and the diffraction effect of this image on the eye. 

 Definition is best with narrow pencils up to a certain point, namely, 

 that at which the narrow illuminating cone being focussed in the 

 optical aperture above the eye piece becomes so small in relation 

 to the magnified image and to the pupil of the eye that diffraction 

 effects begin to appear. Prom this point, any increase of intensity 

 of light of the narrow pencils increases diffraction ; but larger 

 incidence angles of illuminating pencils moderate diffraction. It 



