ADJUNCTS TO THE MICROSCOPE. 43 



concave mirror from the axis* of the instrument, but in this 

 way the illumination is confined to one side of the object while 

 the other side is in shadow. M. Nachet has contrived an 

 oblique prism which can be revolved so as to throw oblique 

 light successively on all parts of the object. Mr. Wenham's 

 illuminator consists of a truncated parabolic mirror (somewhat 

 the shape of the half of an egg-shell with about half an inch of 

 the apex cut off) fastened beneath the stage plate. At the 

 bottom of this mirror is a circular stop of the size of the 

 opening at the other end. This carries a dark well up nearly 

 to the stage plate. When this illuminator is used the light 

 is thrown up by means of the plane mirror, and by reflexion 

 from the parabola is made to pass behind and around the dark 

 well. Direct light is prevented by the circular disc or stop. 

 This is a most admirable contrivance. The objects appear 

 brilliantly illuminated on a dark ground. The illuminator 

 usually has a meniscus at the small end to correct the aber- 

 ration of the slip of glass which carries the object. 



Nobert's illuminator, like the last, throws an oblique 

 light all round an object, and of course there is no shadow. 

 It consists simply of a thick plano-convex lens, in the centre 

 of the convex surface of which a deep concavity is made. The 

 plane side is turned towards the object, and it is placed in a 

 manner similar to the Wollaston Condenser. The concavity 

 in the lens is equivalent to a dark spot on the convex surface, 

 so that a hollow cone of light is obtained, in the apex of which 

 the object is placed. It is necessary to have lenses of different 

 sizes for object-glasses of different focal lengths. 



Polarizing Apparatus (Fig. 14), for viewing objects by 

 polarized light. It consists usually of two prisms of calca- 

 reous spar, in proper tubes; one below the stage, and the 

 other in the eye-piece. Sometimes a thin piece of tourmaline 

 is used in place of the prism in the eye-piece. 



