226 THEORY OF MICROSCOPIC OBSERVATION. 



Finally, we will refer to a point which we think has hitherto been 

 overlooked. Elevations and depressions may be always replaced 

 in transmitted light by differences of density, which produce exactly 

 similar optical effects. If we suppose a membrane of equal 

 thickness throughout, but with dense and loose parts, the former 

 must appear convex and the latter concave, in the stereoscopic 

 image, and consequently, produce an illusive impression on the 

 mind in the endeavour to combine them. Hence we consider it 

 in all cases preferable to conduct scientific researches with the 

 ordinary monocular Microscope. 



III. 



INTERFERENCE PHENOMENA. 

 A. In the Microscope. 



1. DELINEATION OF THE FINE STRUCTURE OF OBJECTS BY 

 INTERFERENCE. 



ACCORDING to the researches of Abbe, 1 which we have already 

 several times referred to, the microscopic image of the object is 

 formed by the superposition of two images, of which the one 

 contains more especially the contours, and is generated in the 

 usual dioptric method ; while the other reproduces the fine details 

 of structure, and is due to the pencils deflected in the object and 

 which interfere in the Microscope. We have now to explain the 

 origin of this interference image and to establish the conditions 

 which influence the power of distinguishing fine details in the 

 object. 



The theory of diffraction shows, as is well known, a character- 

 istic change which light undergoes in its passage through .objects 

 composed of fine structural details. In general terms the change 

 consists in this, viz., that every incident parallel pencil of rays 

 is resolved into a divergent group of rays with large angular 

 extension, in which a regular increase and decrease of intensity 

 is apparent as a necessary consequence of the difference of phase. 

 1 " Archiv fur mikr. Anat." lid. ix. p. 413. 



