232 ox THE THEOKY OF THE MICROSCOPE. 



inexplicable. The result, then, of this preliminary study is to 

 give the following form to the enquiry — namely, to find out the 

 special causes outside the microscope which co-operate in the 

 formation of images of small structural details, and then to deter- 

 mine individually the mode and manner of their intervention in 

 the dioptric process. Each of these requirements has been 

 fulfilled theoretically and experimentally as far as needed for our 

 present purposes. 



XY. The undulation theory of light demonstrates in the 

 phenomena of diffraction or inflection a characteristic change 

 which material particles, according to their minuteness, effect in 

 transmitted (eventually also in reflected) rays of light. This 

 change consists, generally, in the breaking up of an incident ray 

 into a group of rays with increased angular dispersion within the 

 range of which, periodic maxima and minima ofintensity /^?.d., alter- 

 nation of dark and light) occur. For the particular cases of regular 

 lamination, striation, rows of points, and the like, mathematical 

 theory offers a full definition of the phenomenon, which consists 

 in this, that from the rectilinear incident ray there is deflected on 

 each side of it a series of isolated rays, which diverge with regulated 

 angular distances from each other. But these angular distances 

 are for each colour proportional to its wave length, and increase, 

 therefore, in size from violet to red, and are also inversely pro- 

 portioned to the distances between the particles in the object which 

 cause diffraction. When a microscopic preparation possessing the 

 structural peculiarity under consideration is illuminated by a 

 cone of rays thrown by the mirror of the microscope upon it, the 

 light does not enter the objective in the same direct line which it 

 held in its course from the mirror towards the object, because the 

 structure of the object causes a number of deflected and color 

 dispersing rays to be separated from the rectilinear rays; and 

 these deflected rays form larger or smaller angles with the lines 

 of direction of the unchanged rays according to the greater or 

 less minuteness of structure. This class of objects transmit 

 therefore point for point several isolated pencils to the objective, 



