THE AMICI PEISM. 187 



emerge at the plane base of the slip, and be lost to 

 view ; but the peripheral zone of the pencil beyond the 

 numerical aperture I'O will not emerge, but is totally- 

 reflected at the internal surface of the base of the slip, 

 and is seen as a luminous zone surrounding a nearly 

 dark field (the field is not absolutely dark, because of 

 the ordinary reflection of light that takes place before 

 the emergence of the central pencil)." 1 



Method of Using Condensers. Whatever the special 

 form of apparatus, it should always subserve the pur- 

 pose of condensing the light reflected by the mirror to 

 a correct focus upon the object. The light reflected 

 from either the plane or concave mirror should pass 

 through the axis of the condenser, moving at the same 

 time in all directions, and in the axis of the objective, 

 body, and eye-piece of the microscope. The secondary 

 stage should be made to 

 admit of perfect centring 

 and be provided with a 

 racking adjustment. Upon 

 changing the objective, 

 Ross's centring eye-glass 

 must be brought into use 

 to ensure the centricity of 

 the condenser and the 

 body of the microscope. 3 



I he Amici .Prism was 

 originally designed for oblique illumination. It con- 

 sists of a flattened triangular glass prism, the two 

 narrower sides of which are slightly convex, while the 

 third or broadest side forms the reflecting surface. 

 When properly used, it is capable of transmitting a 

 very oblique pencil of light. The prism is usually 

 mounted, as in fig. 120, for slipping into the sub- 

 stage. 



The LieberJcuhn. The concave speculum termed a 



(1) English Mechanic. 



(2) This centring-glass consists of a tubular cap with a minute aperture, 

 containing two plano-convex lenses, so adjusted that the image of the aper- 

 ture in the object-glass, and the images of the apertures of the lenses and 

 the diaphragms contained in the tube which holds the illuminating com- 

 bination, may all be in focus at the same time, so that by the same adjust- 

 ment they may be brought sufficiently near to recognize their centricity. 



