GLASGOW SOCIETY OF FIELD NATURALISTS. 139 



identical property with the parabola. Mr. "Wenham's paraboloid 

 has its apex ground off down to ^-inch below the focus, and a dark- 

 ground stop in the centre. If the light is reflected from the plane 

 mirror or rectangular prism on the paraboloid, an object placed in 

 its focus will appear brilliantly lit up on dark ground. The para- 

 boloid yields a blacker ground than the spot-lens, and the rays 

 passing out of it in a more horizontal direction, it can be employed 

 with the l-5th object-glass. 



Wenham's Reflex Illuminator (Fig. 16) is another most ingenious 

 contrivance for obtaining oblique dark-ground ilkimination under 

 the highest powers, provided the objects to be examined by its aid 

 are suitable ones, attached to the slide and not to the cover, and 

 mounted dry, that is, not in balsam or any other medium. This 

 reflex illuminator is composed of a glass cylinder half-an-inch long 

 and 4-lOths inch diameter, the lower convex surface of which is 

 polished to a radius of 4-lOths inch. The top is flat. Starting 

 from the bottom edge the cylinder is worked ofi" to a polished face 

 at an angle of 64*^. Close beneath the cylinder is set a plain, con- 

 vex lens of 1^ -inch focus. Parallel rays thrown up through the 

 apparatus are reflected from the polished ofi" plane of the cylinder 

 on to the glass-slide, at an angle of total reflection; but if a suit- 

 able object adheres to the surface of the slide, the light reaches it 

 on an angle that admits of its passage. In order to ensure the 

 continuous passage of the rays to the object, the top of the glass 

 cylinder and the lower surface of the slide must be united by a 

 film of water. The object appears then brilliantly lit up upon a 

 dark ground, and when the apparatus is rotated splendid efiects 

 and an exquisite unfolding of structure is obtained. 



Nothing I'emains now but to speak of microscopical illumination 

 with polarized light. I mentioned at the outset that light consists 

 of the undulations of the luminiferous ether, and that these undu- 

 lations take place in two planes at right angles to the ray. The 

 object of the polarization is to bring these two sets of undulations 

 into one plane, and this is efiected by double reflection. An 

 apparatus which polarizes light is called a Folariscope, and the 

 phenomenon of polarization is one of the most gorgeous in 

 nature, and serves at the same time the practical purpose oi 

 enabling the microscopist to form conclusions as to the nature of 

 the object under observation by polarized light. The Micro- 

 polariscope consists of two prisms made of Iceland spar, or 



