129 



These arrangements afford the usual large aperture for throwing a 

 light from the mirror upon a Lieberkuhn, and do not in any way in- 

 terfere with the ordinary diaphragm or the adaptation of the achro- 

 matic condenser. 



In applying this principle to the stages of microscopes much will 

 depend on the taste of the microscopist,.and the ingenuity of the 

 workman, but the chief points to be attended to are, the making the 

 centre of the revolving-plate to coincide with the axis of the instru- 

 ment, and placing the adjustments above the revolving-plate. 



It will be found by experiment that an object placed in the centre 

 of the field under one object-glass will not be exactly so when another 

 is employed ; this arises from the well-known difficulty of accurately 

 centreing the object-glasses, and consequently the almost impossi- 

 bility of making the optical and mechanical centres exactly the same. 

 I may state, however, that the old stage of my microscope has been 

 altered in accordance with the principle above laid down, and I find 

 that under a power of 500 diameters an object is retained in the field, 

 though not in the centre, during the three-fourths of a revolution to 

 which the stage is limited (an approximation to mechanical truth suf- 

 ciently close) that the slightest touch of the adjustments will bring the 

 object again to the centre. 



