74 PRACTICAL MICROSCOPY. 



The light of a paraffin lamp is then to be condensed upon 

 the object through one of the faces of the prism, until it is 

 seen by inspection through the other face that the objective 

 is well illuminated. 



Nachet's prism is shown in Fig. 53, the upper and lower 

 surfaces of which are convex, by reason of which the rays 

 of light proceeding from the mirror are con- 

 veyed by the lower surface, and after under- 

 going two reflections are finally brought to 

 a focus upon the object. The prism should 

 be so mounted that it may be revolved 

 for the same reasons as have been already 

 FlG - given when treating of the Amici prism. 



The Nachet prism is not much used in 

 this country, and in its present form it is constructed with 

 the lower surface plane and not convex as in the original. 

 Then, again, there is the right-angle prism which finds 

 occasional use. The proportion of light reflected by a 

 prism is greater than can be obtained from the ordinary 

 plane silvered mirror, which always yields secondary 

 reflections from the surfaces of the glass. 



Lord Osborne's illuminator or diatom exhibitor is made 

 by Messrs. Baker, of Holborn, and is much praised by many 

 microscopists as an aid in resolving markings on plane 

 surfaces ; it is somewhat similar to Reade's condenser in 

 the form of the lenses, but they are mounted differently. 



The subject of condensers and prisms is one which 

 should be well studied by the beginner before he decides 

 to purchase ; always talk the matter over well with experts 

 before committing yourself to any particular form. 



The illumination of opaque objects under high powers 

 has been attempted by many, but the most successful 

 appliances for this purpose are easily enumerated Powell 

 and Lealand's, and Beck's vertical illuminators, and Tolles' 



