181 PHOTOMICROGRAPHY 



A central stop prevents the passage of light direct to the 

 objective at any angle less than that corresponding to a 

 N.A, of 1 ; beyond this, light is admitted to the illuminator, 

 and reflected at a single surface as in the paraboloids of 

 Zeiss and Watson, or at two spherical surfaces as in the 

 illuminators of Leitz and Swift, up to a N.A. of about 



Fig. 58 



DIAGRAM OF ILLUMINATION BY WATSON'S PARABOLOID 



1-35 or 1-4. Fig. 58 shows the course of rays in the 

 Watson paraboloid. The light diffused from the object is 

 represented as filling the aperture of the objective, but 

 it is not possible to show what becomes of the direct light 

 as each ray is reflected back at the under surface of the 

 cover-glass in exact coincidence with a corresponding ray 

 from the opposite side of the paraboloid, the light, as it 

 were, returning upon itself. 



The reflecting surfaces are so accurately shaped in 

 several of these illuminators that spherical aberrations 

 are practically eliminated, and the rays are brought to 

 a point with great nicety, as indicated in the diagram of 

 the passage of light through the Leitz dark ground 

 illuminator (Fig. 59). As a consequence it is necessary 

 that the microscope-slides used with these condensers be 

 of such thickness that the focus of the rays is in the plane 

 of the object : the required thickness is given with each 

 make of condenser, and is generally about 1 mm. 



