ON OPTICAL aUALITY OF MICROSCOPE OBJECTIVES. 

 Fig. I. 



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

 to extend from the centre of the field to ^V*^ ^'^^^ outside of it ; 

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

 between the gV^h and ^^th inch (measured from the centre) and 

 the third, the peripheral zone, on the same side as the first 3 as in 



figure 2. 



Fig. 2. 



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, 

 of 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 with 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 concidence 

 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 co-incident 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. 



