122 THE MICROSCOPICAL NEWS. 
course of any pencil entering within the angle of aperture of the 
objective to be tested. 
The testing process has for its aim to view the co-operation of 
every zone of the aperture, whether central or peripheral, and yet, 
at the same time, to be able to distinguish and recognise the images 
which each zone delivers separately. For this purpose the illumi- 
nation is so regulated that every zone of the aperture shall be 
represented in the image formed at the upper focal plane by tracks 
of the entering pencils of light, yet so that for each zone a small 
streak only of light be let in, and that the tracks be kept as widely 
apart from each other as possible. If an objective be absolutely 
perfect, all these images should blend wth one setting of focus into 
a single, clear, colourless picture. 
A test image of this kind at once lays bare in all particulars the 
whole state of correction of the microscope. With the aid which 
theory offers to the diagnosis of the various aberrations, a com- 
parison of the coloured borders of the separate partial images, and 
an examination of their lateral separation and their differences of 
level, as well in the middle as in the peripheral zones of the entire 
field, suffice for an accurate definition of the nature and amounteof 
the several errors of correction, each of them appearing in its own 
primary form. 
Assuming the theoretical knowledge and practical experience 
necessary to carry out such an inquiry properly, and to estimate 
its results correctly, the mode of procedure above described affords 
so exhaustive an analysis of the qualities of an objective, that when, 
in addition, its focal length and angle of aperture are ascertained, 
its whole capacity of performance may be determined beforehand. 
Whoever has once examined in this manner even good objectives 
which have proved to be excellent in practice, will be as little dis- 
gosed to accept childish assertions of their perfectness as to advance on 
his part absurd pretensions which no one has yet made good. 
That the performance of the microscope does not always depend 
solely on the geometrical perfection of the image, but also, in addi- 
tion to this, in certain classes of objects, upon amount of angular 
aperture, is a fact long recognized. The exact significance of this 
fact has nevertheless remained as problematical as the exact nature 
of the quality of “ resolving” or discriminating power. It remained 
a question, What value might be assigned to the quality thus related 
with angular aperture, and does its significance extend any farther 
than to certain cases in which shade effects were supposed to be 
produced by oblique illumination ? 
In the endeavour to establish a theoretical basis for the 
construction of the microscope, it was a matter of the first 
importance to define the exact function of angular aperture in 
the normal performance of the microscope, lest I should fall into 
