210 ON THE THEORY OF THE MICttOSCOPE. 



SO far as it is produced by the objective alone, lies in or close to 

 the upper focal plane of the objective, where also it may be seen 

 by looking down the tube of the microscope with the naked eye. 

 These two sets of images are interconnected by common relations, 

 the determination of which affords a key to the solution of 

 questions scarcely to be approached by any other means. All the 

 characteristics of the object images hang together with certain 

 characteriaticPy of the aperture images, and vice versa. Those of 

 the aperture image, in particular, afford the means of determining 

 the limiting outlines of the pencils of rays by which images of 

 the object are formed. On this again are based, theoretically, 

 propositions of general application respecting the so called 

 *' penetrating power" of the microscope, and respecting the 

 influence which diffraction of light passing in through the 

 aperture of the objective exerts upon the microscopic picture ; and 

 above all, respecting the conditions which affect the brightness of 

 the picture, and the modus operandi of the several methods 

 of illumination, and the various appliances for this purpose. On 

 the other hand, actual observation of these aperture images, 

 conducted with suitable apparatus, affords additional means 

 of studying the object, because in these aperture images the track 

 of every ray which, coming from the object, enters the microscope 

 in any direction whatever, will be seen. For instance, the bright 

 parts of any *' aperture image " {e.g., the first one formed, after 

 passing through the lens in the upper focal plane of the objective) 

 indicates the several pencils which come from the obj ect and form the 

 image. Hence any change produced by the action of the substance 

 of the object itself on these rays, especially any deflection of rays,- 

 will be at once recognised by some corresponding change of the 

 aperture image. This fact will find manifold applications, and be 

 further discussed in the following chapter. 



The principle on which is founded the full development of 

 the study of these aperture images leads to various results, 

 depending for their full development upon a principle which 

 constitutes at the same time a law of fruitful application 



