V 



174 THE MICROSCOPE. 



The parabola answers quite well for objects in balsam 

 or mounted dry, but its application scarcely extends 

 to object-glasses higher than l-5th, unless of large 

 aperture. 1 



Wenham's parabolic reflector, seen in section, fig. 

 106, a a, of a tenth of an inch focus, has a polished 

 silver surface, the apex of which is cut away so as 

 to bring the focal point at a proper distance above the 

 top of the apparatus (which is closed with a screw- 

 cap when not in use), thus allowing the pencil ot 

 light to pass through the thickest glass cover used for 

 mounting. At the base of the parabola is a disc of 

 thin glass b &, in the centre of which is cemented 

 a dark well, with a flange equal in diameter to the 

 aperture at the top of the reflector, for the purpose of 

 stopping all direct rays from passing. 



The reflector is moved to and from the object by 

 means of the rack and pinion c, with a similar adjust- 

 ment for centring, and is either fixed under the stage 

 of the microscope or made to slide into the sub-stage ; 

 in addition, there is a revolving diaphragm d, with two 

 apertures e e, placed diametrically, for the purpose of 

 obtaining two pencils of oblique light in opposite 

 directions. 



In using the paraboloid, the plane mirror is so ad- 

 justed that parallel rays enter it and impinge on the 

 rabolic sides of the reflector, in such a manner as to 

 totally reflected without suffering refraction, and 

 meet in the centre of a spherical hollow made in the 

 top of the paraboloid. The adjustable stop being 

 either raised or lowered, will effectually arrest all 

 superfluous rays. 



The light most suitable for this method of illumina- 

 tion is lamp, the rays of which should in all cases be 

 rendered more parallel by means of a large plano-convex 

 lens, or condenser. 



The Immersion Illuminator. Mr. Wenham, in the 

 year 1856, described various forms of oblique illumina- 

 tors, one of which was an immersion ; a simple right- 



(1) See an excellent summary of the value of parabolic illumination and 

 immersion illuminators by Mr. J. Mayall, junr., Vol. II., p. 27, Journal of 

 the Royal Microscopical Society (1879). 



