ox THE THEOKY OP THE MICKOSCOPE. 247 



possille operation. But its actual amount will, at the same time, 

 depend upon the exactitude with which the partial images 

 corresponding to the respective diffraction pencils blend together : 

 for it is through this last act, that the detail which indicates the 

 existence of positive structural elements in the object is rendered 

 visible. ]S"ow, inasmuch as these isolated pencils, whose confocal 

 reunion is the necessary condition of the formation of diffraction 

 images, occupy different parts of the aperture, and vary constantly 

 in position according to the character of the object and the mode 

 of illumination: it is obvious that a perfect fusion, in every case, 

 of the several diffraction images, and then an exact super position 

 of the resultant "diffraction image" upon the "absorption image," 

 is only possible when the ohjedive is uniformly free from spherical 

 aherration over the whole area of its aperture. 



XXI. According to the authoritative representations hitherto 

 given of the process by which an image is formed in the 

 microscope, one might suppose that residual aberrations in the 

 objective only impair the sharpness of definition, and that such 

 aberrations either did not exisl, or might be considered, as 

 practically, of no importance so long as there was no visible 

 failure of definition. But the facts shewn above, taken in 

 connection with what has been said (paragraph vii.) of the typical 

 form of spherical aberration in objectives of large angular aperture, 

 place their significance in a very different light. The several 

 elements of the microscope image, namely, the ^^ absorption image,''^ 

 and the several constituent ^^diffraction images''^ are produced by 

 isolated pencils of relatively small angle of divergence scarcely 

 ever beyond 30^ to 40^. And even with a considerable residual 

 spherical aberration, the points of such isolated pencils, each 

 considered by itself, are sharp enough to leave scarcely any 

 noticeable dispersion circle. As however, with a large aperture 

 these separate pencils operate through very different parts of the 

 aperture at the same moment, their focal points cannot re-unite 

 if the residual spherical aberration is considerable, but must 

 appear beside or behind each other. Hence the component parts 



