17° THE MICROSCOPICAL NEWS. 
image, in which the several constituent parts of an object re-present 
themselves geometrically, by virtue of the unequal emergence of 
light which is caused by their mass affecting unequally the trans- 
mission of the incident rays. This image may, for shortness sake, 
be called the “ absorption image,” because partial absorption is the 
principal cause of the different amount of emergent light. It is 
the bearer of the “ defining” power, whose amount is determined 
by the greater or less exactitude with which direct incident light is 
brought into perfect homofocal reunion. Consequently, it is always 
the drect light which “ defines,” no matter in what directiort it 
arrives at the objective, z.¢., whether the central or peripheral zones 
of the objective receive it. But, indepertdently of the “ absorption 
image,’ all such parts of the object as contain interior structure 
will be imaged a second time, and this time as a fosétive image, 
because these parts will appear as if self-luminous, in consequence 
of the diffraction phenomena which they cause. Now this “diffraction 
image” is manifestly the bearer of “resolving” power, that is, the 
discriminating or separating faculty of the microscope. Its develop- 
ment depends, therefore, in the first and chief place upon angular 
aperture, in so far as this alone determines, according to rules 
above given, the “mits of its possible operation. But its actual 
amount will, at the same time, depend upon the exactitude with 
which the partial images 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. Now, inasmuch as 
these isolated pencils, whose confocal reunion is the necessary con- 
dition 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 superposition of the resultant “ diffraction image” 
upon the “absorption image,” is only possible when the objective ts 
uniformly free from spherical aberration over the whole area of tts 
aperture. 
In consequence of the onesidedness with which, in modern times, 
the improvement of the microscope has been directed towards the 
increase of angular aperture, the conditions under which abnormal 
appearances, and especially deceptive alterations of level are pro- 
duced, occur abundantly in the new high-power objectives, as 
repeated experience has shown me, and I assuredly do not err in 
expressing my conviction that the consequences of this state of 
things affect to an unexpected extent the numerous questions in 
dispute amongst microscopists concerning the interpretation of 
minute structures. 
Since everyone must admit that the first and most imperative 
claim which can be made, in the interest of scientific microscopy, 
