20 PROPERTIES OF LENSES. 



ing image as regards the actual structure of the object.* 

 These diffraction phenomena must not be con- 

 founded with those appearances surrounding the 

 outlines of. an object, mentioned in a previous para- 

 graph when speaking of correction for cover glass. 



It has been recently remarked that microscopic 

 vision was totally distinct from macroscopic. First, 

 because by placing a stop behind the objective that 

 is decreasing the aperture, objects could be made to 

 assume different forms. Second, because under dif- 

 ferent conditions of aperture different objects as- 

 sumed the same form. 



Now Professor Abbe himself says " minute struc- 

 tural details are not as a rule imaged by the micro- 

 scope dioptrically , in accordance with the real nature 

 of the object/' 



Some, less enamoured of the metaphysical aspect 

 of the question, may find it difficult to perceive that 

 images formed optically and dioptrically give us just 

 as little information as to the real nature of the 

 object. If two scarlet cloths the same size, one marked 

 with black squares the other with circles, and one 

 coarser than the other, be placed at such a distance 

 that their images subtend small angles, they will 

 appear identical. If brought nearer, or what is the 

 same thing, made to include a larger angle, a greater 

 number of rays are collected and certain differences 

 appear, and as the angle of vision increases, the 

 structural peculiarities still continue to cause fresh 

 visual phenomena. 



* The Italics are my own. 



