122 SUMMAEY OF CURRENT RESEARCHES RELATING TO 



others only, but by itself and on its own merits, affords not only the 

 most direct and positive evidence of its qualities to those who are 

 more concerned in proving their instruments than using them, but 

 also yields to the genuine worker the satisfying conviction that his 

 labour is not frustrated by faulty construction and performance of his 

 instrument. It is, however, to be borne in mind that the microscopist, 

 in any scrutiny of the quality of his lenses which he may attempt, has 

 no other object in view than to acquire such insight into the optical 

 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 manijiulative skill in examining microscopical 

 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 ex- 

 haustive 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 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 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 objec- 

 tive to be l-4th in. in diameter, the image of the pencil of light let in 

 should not occupy a larger space than 1-1 6th in. When two pencils 

 are employed, one of these should fall so as to extend from the centre 

 of the field to l-16thin. 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 l-16th in. clear between its own inner margin and the 

 centre of the field, as in fig. 16, where the objective images of the 

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



* Arch. f. Mikr. Anat., ix. (1873) p. 413. 



