454 ON APERTUEE AND DEFINITION OF MICROSCOPE OBJECT GLASS. 



is, however, clear that aperture is not aloiie concerned in affecting 

 the minimum of diffraction with maximum definition, since the 

 result depends also upon the relative amplification of image 

 (magnifying power), the size of detail in the object, and the nature 

 of illumination, i.e. kind of light, and whether central or oblique 

 illumination. JSTevertheless, cater is paribus wider angle offers a 

 wider range of manipulation and effect to the practised 

 microscopist. 



In respect to the second point, the greater inclination of pencils 

 to the axis, the advantage arising therefrom lies chiefly in the 

 power of varying the illumination. The widest cone of light that 

 can enter an objective is that whose point lies on the object exactly 

 in the centre of the axis of the instrument, and whose divergence 

 just fills the available aperture. This cone (above the object) 

 requires that the illuminating cone below the object should be of 

 equal size, and the regulation is best effected for each separate 

 objective by diaphragm and illuminating lens system. Such a mass 

 of light collected equally from all sides must of course yield the 

 utmost quantity that can be obtained, and is, therefore, needed to 

 give brightness to the wide spread image of an object magnified by 

 the highest powers. If there be no chromatic dispersion (which, as 

 well as spherical aberration, is a defect inherent in large aperture), 

 this wide cone of light, entering with relatively wide divergence 

 angle, tends to moderate the diffraction associated with highly 

 magnified images. But, excepting cases of enormous amplification, 

 this absolute maximum of light defines badly, because the bright 

 images formed by pencils passing through every part of the 

 periphery of aperture pour from all sides such a flood of light upon 

 each other as to lessen the delineation by the darker outlines of the 

 absorption (negative) picture. Besides which the general bright- 

 ness of the field fatigues the retina and also occasions entoptic 

 shadows. 



On the other hand, if this central illumination be shut off on 

 one side, or a central stop used with it, a lateral or peripheral 

 illumination with partial or entirely dark field offers the opportunity 



