ON THE THEORY OF THE MICROSCOPE. 255 



place. We have the outline image of the object as formed by 

 the direct light, and, at the same time, a detail image of content, 

 which is produced by the interference of diffractive rays. Wlien 

 the objective is corrected properly, each image stands out sharply 

 defined by itself, whilst both fall accurat-^ly together without 

 difference of level in any lateral displacement —that is to say, both 

 images are clearly seen, with one setting of focus, when in the 

 object itself outline and structure are on the same level. If a 

 system of lenses works satisfactorily under a trial of this kind, 

 carried out with a few turns of the fine movement screw, or, at 

 lea^t, if it performs well in the middle of the field, we may be 

 sure that it will always give correct images of any object, and 

 with any kind of illumination. On the contrary when — the 

 objective being focussed for outline image — details appear to 

 hover above or beneath it, or float away, sidewards, from it, a 

 construction of objective is indicated, which offers no certainty 

 that markings which belong together in any given preparation 

 will be recognised as belonging together in the microscope image, 

 however highly the '' resolving power " may exhibit itself with 

 the usual mode of testing it. 



Without limiting the mode of illumination to the two positions 

 already described, the judgment may be assisted in various points 

 when other positions of the mirror are tried and their effect 

 pro\ ed, attention being always directed to the characteristic signs 

 of fusion of the partial^ images. But in every trial it is scarcely 

 necessary to observe that the effective course of the rays must be 

 controlled by direct observation of the aperture images. 



In all large-angled objectives, deviations of the kind alluded to 

 will be observed in the outer circle of the field of vision, unless 

 the visual angle of the eyepiece be unusually small. They arise, 

 not from aberration, but chiefly from differences of amplification, 

 which are unavoidable even in the best objectives. The extent 

 to which they occur is the measure of relative imperfection of 

 the image formed outside the axis. What further belongs to our 

 judgment of the good qualities of a system of lenses may be 



