252 Extracts from Mr. II. E. Fripp's Translation of 
as this alone determines, according to rules above given, the limits 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 indi- 
cates the existence of positive structural elements in the object is 
rendered visible. Now, inasmuch as these isolated pencils, whose 
confocal reunion is the necessary condition of the formation of dif- 
fraction 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 “absorp- 
tion image,” is only possible when the objective is uniformly free 
from spherical aberration over the whole area of its 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 ab- 
normal appearances, and especially deceptive alterations of level are 
produced, 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, 
upon the performing power of the instrument is this — that parts 
which belong together in the object shall also appear as belonging 
together in the microscopic image, it follows that uniform correc- 
tion of spherical aberration throughout the whole area of aperture 
must be the absolute criterion and rule of guidance in the construc- 
tion of a microscope. Now, it has been shown that with a dry 
objective an adequate compensation of spherical aberration is, as a 
matter of fact, impossible when the angular aperture exceeds 110°. 
Hence it must be concluded that a dry objective will be less suited 
for ordinary scientific use in proportion as it renders visible such 
finer systems of lines as exceed the limits of resolving power 
answering to that angle (namely, O' 35 /a for oblique light). The 
greatest possible increase of resolving power can be obtained in a 
rational way only by means of immersion objectives, as these alone 
admit of the largest possible (i. e. technically practicable angular) 
aperture, without contravening the very first requirement of cor- 
rected spherical aberration.* 
* The dry objectives made on Abbe’s calculations, founded upon the princi- 
ples before explained, have only 105° to 110° of angular aperture for the highest 
powers, and cannot pretend therefore to compete, in resolving diatoms, &c., with 
