176 THE MICROSCOPE. 



on the slide, found by the aid of a low power, and 

 distinguished by their brilliancy, or by rotating the 

 illuminator ; the effect on the Podura is superb, the 

 whole scale appearing dotted with bright blue spots 

 in a zig-zag direction. Objects for this illuminator 

 should be specially selected or mounted on the slide. 



Mr. J. May all, Jun.'s, semi- cylinder or prism for 

 oblique illumination (fig. 108) is a convenient form, 

 as it permits of the semi-cylinder 

 being tilted and placed excentrically ; 

 in this manner, without immersion 

 contact, and by suitable adjustment, 

 a dry object can be viewed with 

 any colour of monochromatic light. 

 If placed in immersion contact with 

 the slide, the utmost obliquity of in- 

 cident light can be obtained. Objects 

 in fluid may be placed on the plane- 

 surface of the semi-cylinder, and 

 illuminated by ordinary transmitted 

 light, or rendered " self-luminous " 

 in a dark field, as with the hemi- 

 spherical illuminator or Wenham's 

 immersion paraboloid. A concave 

 mirror with a double arm is quite 

 sufficient to direct the illuminating pencil. This semi- 

 cylinder was originally made by Tolles, of Boston, for 

 measuring apertures, but, at Mr. Mayall's suggestion, 

 Messrs. Ross mounted it as an illuminator. 



The Achromatic Condenser. 



The aim of the microscopists in bringing the achro- 

 matic condenser into use, is to secure a pencil of light 

 that shall approximately fill the aperture of the objec- 

 tive, and by the intervention of central stops, or slots, 

 the various portions of the cone of condensed light, 

 according to the kind of object under examination, shall 

 fully utilize the same. 



The peculiar advantages of employing an achromatic 

 condenser for the purpose indicated, were first pointed 

 out by Dujardin, since which time an object-glass 



FIG. 108. Mayall's 

 Semi-Cylinder 

 Illuminator. 



