ON OPTICAL aUALITY OF MICROSCOPE OBJECTIVES. 



which, it is hoped, will enable the reader to make satisfactory 

 trials for himself. 



The process is based on the following principle : 



In any combination of lenses of which an objective system is 

 composed, the geometrical delineations of the image of any object 

 will be more or less complete and accurate as the pencils of light 

 coming from the object are more or less perfectly focussed on the 

 congugate 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 the 

 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. 



I. 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, {i.e., close behind or above 

 its back lens,) by a pencil of light isolated in its passage through 

 the lens, so that only one narrow track of light be allowed to pass 

 for each zone, the tracks 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 i inch in diameter, the image of the pencil of 

 light let in, should not occupy a larger space than -^^ inch. When 

 two pencils are employed, one of these should fall so as to extend 

 from the centre of the field to ^^g-th inch 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 ^^\h inch clear 

 between its own inner margin, and the centre of the field, as in 

 figure I. 



Where the objective images of the pencils occupy each a 

 quarter of the diameter of the whole field. 



