58 On Dark-grotmd Illuniination, [Sess. 



perfectly opaque by a coating of dead -black varnish. On 

 throwing parallel rays upon the under side of this lens, the 

 central ones strike the opaque surface and do not reach the 

 object ; but the marginal ones that fall outside the stopped 

 centre or spot are transmitted, refracted, converge, and come to 

 a focus, which focus may be made to rest upon the object by mov- 

 ing the lens up or down. Now these marginal rays diverge, after 

 passing the focal point, in the same way as those we have 

 already considered, but with this difference, that they pass 

 onwards on each side of the olijective. They, however, do not 

 enter the tube, consequently the reflected rays only are seen on 

 looking through the microscope, so that the object thus illumin- 

 ated appears upon a dark ground. This method is better than 

 the first, because both of the transmitted beams pass at equal 

 angles to the axis of the microscope tube — or in other words, 

 the axes of the objective and the spot-lens are coincident. On 

 account of this the spherical corrections of the lenses are not so 

 severely taxed ; and the diffraction, or fringe of light in the 

 object, is not rendered eccentric, but is better distributed round 

 the edges. 



III. By Means of the Ahhe Condenser. — Most of you will 

 know that this is the form of condenser almost exclusively used 

 in Continental microscopes. It is a non-achromatic combina- 

 tion ; and whether or not the want of achromatism is against 

 its usefulness as a sub-stage condenser, certain it is that, as a 

 dark-ground illuminator for diatoms, its performance is not 

 thereby impaired. Like the spot-lens, it is lit up by rays from 

 the mirror underneath the stage, the central rays being cut off 

 by an opaque surface. There is, however, a decided difference 

 in the way the exclusion is effected. Underneath the Abbe 

 combination a circular central stop is inserted, so that the 

 cutting off takes place in the centre of the entrant rays, whereas 

 in the case of the spot lens the stop takes effect upon the 

 emergent rays. Owing to the peculiar construction of the 

 apparatus, the rays are brought to a focus almost at their 

 emergence from the upper lens, and thus a much closer working 

 distance is available ; or in other words, a higher power may 

 be used without admitting transmitted rays into the objective. 

 By this instrument the size of the stop can be regulated, and 

 the more intense beam gives brighter illumination. The 



