384 OBJECTIVES, EYE-PIECES, THE APERTOMETER 



conditions of good performance as will enable him to make the best 

 use of his instrument, and acquire confidence in his interpretation 

 of what he sees, as well as manipulative skill in examining micro- 

 scopical objects. To the constructor and expert of optical science 

 are left the severer investigations 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 

 ^ind lie entirely outside the sphere of the microscopist's usual 

 work. 



Professor Abbe's mode of testing objectives is explained in his 

 4 Beitrage zur Theorie des Mikroskops.' 



The process, in our judgment, requires large experience and 

 much skill to be of practical service ; but it 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 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. 



1 . 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), 

 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 ^ 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 ^ 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 /,.; inch clear between its own 

 inner margin and the centre of the field. The objective images of 

 the pencils occupy each a quarter of the diameter of the whole field. 



If three pencils of light be employed, the first should fall so as to 

 extend from the centre of the field to ^ inch outside of it ; the 

 second should occupy a zone on the opposite side of it, between the 

 J- and -jL inch (measured from the centre) ; and the third the 

 peripheral zone on the same side as the first in fig. 329. 



This arrangement places the pencils of light in their most sensi- 

 tive position and exposes most vividly any existing defect in correc- 

 tion, since the course of the rays is such that the pencils meet in 



