ON THE THEORY OF THE MICROSCOPE. 209" 



Section II. — The dioptric conditions on which tJie performance of 

 the microscope depends. 



Por the purpose of demonstrating the course of rays of light by 

 which images of an object are formed in the microscope, a well- 

 known diagram is usually employed, which shews, constructively, 

 a reversed image produced by the objective and collected by the 

 field lens of the ocular, which image is further enlarged and 

 projected by the action of the eye-lens to the distance of clear 

 vision in front of the observer's eye. And the discussion of 

 each special condition, upon which the resulting total optical effect 

 quantitative as well as qualitative depends, is based upon analysis 

 of what takes place according to this diagram of construction. 

 Such a scheme may perhaps suffice to give a general idea of the 

 operation of the microscope; bat if dioptric analysis be required 

 for a more strict estimate and examination of the several factors 

 concerned in the image-forming process, it must be carried out 

 more completely, and extended in new directions. 



ET. The course of the rays must be followed from a more 

 general point of view. The same rays which, coming in 

 homofocal pencils from each constituent point of the object, enter 

 the microscope to form an image of that object, must also be 

 regarded as homofocal rays coming from the several points of a 

 plane situate outside the microscope (before or below it). This 

 plane is, dioptrically interpreted, the outwardly projected aperture 

 of the objective, and includes any object within the outlines 

 marking the angle of aperture; in particular, the source of 

 light serving for illumination of the object, e.g.y surface of 

 mirror. Consequently, in addition to those images of the 

 object which are thrown off successively by the lenses of the 

 microscope, a series of associated images of the aperture are 

 simultaneously thrown off, which together form an image of the 

 outwardly projected plane of aperture. This latter (aperture 

 image) is thus associated with the final virtual image of the object, 

 and appears at the eyepoint, so called, above the ocular where it 

 may be examined with a lens. But the image of the object. 



