ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 123 



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

 extend from the centre of the field to l-25th in. outside of it ; the 

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

 l-25th and 1-1 2th in. (measured from the centre), and the third, the 

 peripheral zone on the same side as the first, as in fig. 17. 



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 Fig. 16. 

 angle. As many distinct images will be perceived as there 

 may be zones or portions of the front face of the ob- 

 jective put in operation by separate pencils of light. If 

 the objective be perfect all these images should blend 

 loith one setting of focus into a single clear colourless ^ ^^- ^'• 

 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 spherical aberration, do not 

 allow this coincidence 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 necessarily be multiplied when the whole area of an objective of 

 faulty construction is in action. 



2. The means hy which such isolated pencils can he obtained. 



If a special illuminating apparatus be employed, the condenser of 

 Professor Abbe will be found very convenient, but almost any con- 

 denser of the kind (hemispherical lens) may be arranged for this 

 purpose. 



In the lower focal plane of the illuminating 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 (i. e. 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 in a card, in such position as will produce images 

 situate in the field as shown by figs. 16 and 17, and the card is then 

 fixed in its place below the condenser. If the condenser be fitted so 

 as to revolve round the axis of the instrument and also carry with it 



