I1o THE MICROSCOPICAL NEWS. 
This arrangement places the pencils of light in their most 
sensitive position, 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 perceived as there may be zones 
or portions 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 wth one setting of focus into a single 
clear colourless picture. Such a fusion of images into one is, how- 
ever, prevented by faults of the image-forming process, which, so 
far as they arise from spherical aberration, do not allow this coin- 
cidence 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 
colour, produce coloured 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 corresponding coincident images in other 
parts of the field. It is to be borne in mind that the errors which 
are apparent with two or three such pencils of light must neces- 
sarily be multiplied when the whole area of an objective of faulty 
construction is in action. 
2. Lhe 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 (hemispherical lens) may be arranged for 
this purpose. 
In the lower focal plane of the illuminating lens must be fitted 
diaphragms (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 (7.¢., of the field seen when looking down the 
tube of the instrument, after removing the ocular, upon the 
objective 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 diaphragms -having holes 
of various 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 objective 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 must then be pierced ina card. If the condenser be fitted 
so as to revolve round the axis of the instrument, and also carry 
with it the ring or tube to which the card diaphragm is fixed, the 
pencils of light admitted through the holes will, by simply turning 
the condenser round, sweep the face of the lens in as many zones 
as there are holes. Supposing the condenser to be carried ona 
