ON THE THEORY OF THE MICROSCOPE. 227 



isolated pencils may be correlated. These would be so arranged 

 that, supposing the surface of the front lens of objective measured 

 J inch diameter, and two pencils were to bo compared, then, in 

 the first case, one track (a nearly circular pencil) would reach 

 from the centre of the field to about ^^ inch from it, the other 

 track, let fall on the opposite side of the axis, would reach the out- 

 side edge of the the aperture occupying a space ^^g- inch distant 

 from the centre to the edge. In the second case, (where three 

 finer pencils are employed) the first track should occupy the zone 

 from centre to a distance of o^jth inch from the centre, the second 

 track a zone on the opposite side between ^Uth and ^\th inch 

 from the centre, and the third track, the peripheral zone, on the 

 same side as the first track. 



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 zones or portions of the objective as are put in 

 operation by the passage of pencils of rays so many distinct 

 images will there be perceived of the group of lines in the 

 prepared object which occupies the field. If an objective be 

 absolutely 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 spherical aber- 

 ration, do not allow this coincidence of several images from 

 difi'erent 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 

 object and the edges of each separate image, as also of the 

 corresponding coincident images in other parts of the field. 



A test image of this kind at once lays bare in all particulars the 

 whole state of correction of the microscope. "With the aid which 

 theory offers to the diagnosis of the various aberrations, a 

 comparison of the colored borders of the separate partial images, 



