1883.] 



MICROSCOPICAL JOURNAL. 



183 



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 di- 

 ameter, the image of the pencil of 

 light let in should not occupy a larger 

 space than -^ inch. When two pen- 

 cils are employed, one of these should 

 fall so as to extend from the centre 

 of the field to iV i"ch outside of it ; 

 and the other should fall on the oppo- 

 site side of the axis, in the outer peri- 

 phery of the field, leaving thus a space 

 of jV inch clear between its own inner 

 margin and the centre of the field, as 

 in fig. 32, where the objective images 



Fig. 32. 



of the pencils occupy each a quarter 

 of the diameter of the whole field. 



If these pencils of light be em- 

 ployed, the first should fall so as to 

 extend from the centre of the field to 

 ^j^ inch outside of it ; the second 

 should occupy a zone on the opposite 

 side of it, between the oV ^"^ yV inch 

 (measured from the centre), and the 

 third, the peripheral zone on the 

 same side as the first, as in fig. ^;^. 



Fig. 33 



This arrangement places the pencils 

 of light in their most sensitive posi- 

 tion, and exposes most vividly any 

 existing defect in correction, since 

 the course of the rays is such that the 

 pencils meet in the focal plane of the 

 image at the widest possible angle. 

 As many distinct images will be per- 

 ceived as there may be zones or por- 

 tions of the front face of the objective 

 put in operation by separate pencils 

 of light. If the objective be perfect 

 all these images should blend with 

 one setting of focus into a single clear, 

 colorless picture. Such a fusion of 

 images into one, is, however, prevented 



by faults of the image-forming process, 

 which, so far as they arise from spher- 

 ical aberration, do not allow this co- 

 incidence of several images from 

 different parts of the field to take 

 place at the same time, and so far as 

 they arise from dispersion of color, 

 produce colored fringes on the edges 

 bordering the dark and light lines of 

 the test object, and the edges of each 

 separate image, as also of the corres- 

 ponding coincident images in other 

 parts of the field. It is to be borne 

 in mind that the errors which are ap- 

 parent with two or three such pencils 

 of light, must necessarily be multi- 

 plied when the whole area of an ob- 

 jective of faulty construction is in 

 action. 



2. T/ie means by which such isolated 

 pencils can be obtained. If a special 

 illuminating apparatus be employed, 

 the condenser of Professor Abbe will 

 be found very convenient, but almost 

 any condenser of the kind (hemispher- 

 ical lens) may be arranged for this 

 purpose. 



In the lower focal plane of the il- 

 luminating lens must be fitted dia- 

 phragms (easily made of blackened 

 cardboard) pierced with two or three 

 openings of such a size that their 

 images, as formed by the objective, 

 may occupy a fourth or sixth part of 

 the diameter of the whole aperture, 

 (/'. e. of the field seen when looking 

 down the tube of the instrument, after 

 removing the ocular, upon the objec- 

 tive image). The required size of 

 these holes, which depends, firstly on 

 the focal length of the illuminating 

 lens, and secondly, on the aperture of 

 the objective, may be thus found. A 

 test object being first sharply focussed, 

 card diaphrams having holes of vari- 

 ous sizes (two or three of the same 

 size in each card) must be tried until 

 one size is found, the image of which 

 in the posterior focal plane of the ob- 

 jective shall be about a fourth to a 

 sixth part of the diameter of the field 

 of the objective. Holes having the 

 dimensions thus experimentally found 

 to give the required size of image 



