ACCESSORY INSTRUMENTS 151 



manner on to the lantern front in place of the ordinary 

 objective, and is supplied with a secondary condensing lens 

 behind the stage. For those not accustomed to optical adjust- 

 ments, it is better that this lens should be fixed, making 

 adjustment of the light as far as possible by hand, with the 

 lime-tray alone. For those capable of using it, however, it 

 is better to have this condenser fitted in either a sliding or 

 rack-and-pinion adjustment, both of which forms are supplied. 

 Then by pushing this lens close up to the slide, the illumina- 

 tion resembles that of the lantern condenser itself, and covers 

 a field nearly equal to its own surface say a circle of 1 inch 

 diameter ; while by drawing it back till it focusses upon the 

 slide, a smaller surface is 

 much more brightly illu- 

 minated. The stage, in this 

 form of attachment, is en- 

 tirely open, and consists of 

 a rotating diaphragm-plate 

 pierced with apertures of 

 appropriate sizes for the 

 various objects, which are 

 held by simple clips. The FW. so 



objective fitting is screwed 



for ordinary microscopic objectives, any of which may be tried, 

 and used if found sufficiently flat in field. If none such can be 

 found amongst the owner's own objectives, the most suitable 

 powers are the 2J inch, 1J inch, and T 8 j inch from the list of 

 the larger instrument described in Chapter XIII. Fair results 

 can be obtained with powers as high as - 5 inch. 



Since its first production, this attachment has found such 

 general acceptance, that some form closely resembling it can 

 now be obtained of almost any respectable optician, with or 

 without certain modifications presumed to be improvements. 

 Of most of these the reader may be left to judge for himself ; 

 but he should be distinctly cautioned against one, viz. a 



