PROFESSOR ABBE’S METHOD OF TESTING OBJECTIVES. 10g 
gations of optical effects and causes, the difficulties of technical 
construction, the invention of new lens-combinations, and the 
numerous methods of testing their labours by delicate and 
exhaustive processes which require special aptitude, and lie entirely 
outside the sphere of the microscopist’s usual work. 
The mode of testing the optical power of an objective here 
described is that devised by Prof. Abbe, and explained in his 
“ Beitrage zur Theorie des Mikroskops.”* 
The process is based on the following principle :— 
In any combination of lenses of which an objective is composed, 
the geometrical delineations of the image of any object will be 
more or less complete and accurate according as the pencils of 
light coming from the object are more or less perfectly focussed 
on the conjugate focal plane of the objective. On this depend 
fine definition and exact distribution of light and shade. ‘The 
accuracy of this focussing function will be best ascertained by 
analysing the course of isolated pencils directed upon different 
parts, or zones, of the aperture, and observing the union of several 
images in the focal plane. For this purpose it is necessary to 
bring under view the collective action of each part of the aperture, 
central or peripheral, while at the same time the image, which 
each part singly and separately forms, must be distinguishable and 
capable of comparison with the other images. 
r. The illumination must, therefore, be so regulated that each 
zone of the aperture shall be represented by an image formed in 
the upper focal plane of the objective (¢.e., close behind or above 
its back lens), so that only one narrow track of light be allowed to 
pass for each zone, the tracts representing the several zones being 
kept, as far as possible, apart from each other. 
Thus supposing the working surface of the front lens of: an 
objective to be 1-4th in. in diameter the image of the pencil of 
light let in should not occupy a larger space than 1-16th in. 
When two pencils are employed, one of these should fall so as to 
extend from the centre of the field to 1-16th in. outside of it, and 
the other should fall on the opposite side of the axis, in the outer 
periphery of the field, leaving thus a space of 1-16th in. clear 
between its own inner margin and the centre of the field, where 
the objective images of the pencils occupy each a quarter of the 
diameter of the whole field. 
If ¢hree pencils of light be employed, the first should fall so as to 
extend from the centre of the field to 1-25th in. outside of it ; the 
second should occupy a zone on the opposite side of it, between 
the 1-25th and 1-12th in. (measured from the centre), and the 
third, the peripheral zone on the same side as the first. 
* Arch. f, Mikr, Anat., ix. (1873) p. 413. 
