ZENTMAYER'S AMERICAN CENTENNIAL STAND. 37 



is in the axis of revolution of the graduated platform, 

 and the angular aperture of an objective focused on this 

 object can easily be measured. It is equally obvious 

 that in this position the object is in the centre of all 

 the revolving parts of the instrument, to wit, the re- 

 volving stage, swinging sub-stage, and the platform. 



The principal stage is similar to the circular one pre- 

 viously used on. the " Grand American;" it is provided 

 with adjusting screws for accurate centring, and re- 

 volves in a large outside ring, giving facilities for 

 oblique illumination up to 70 degrees from axis (140 

 degrees aperture), while the graduations serve as a 

 goniometer for the measurement of crystals, etc. 



The sub-stage is divided into two cylindrical receivers, 

 to facilitate the adaptation of several accessories at one 

 and the same time ; the lower cylinder can be moved up 

 and down or entirely removed. 



The fine adjustment (in all other instruments of the 

 Jackson model being in front of the body) is removed 

 to the more stable part of the stand. The bar is pro- 

 vided with two slides, one for the rack and pinion 

 movement, and close to it another one of nearly the 

 same length for the fine adjustment, moved by a lever 

 concealed in the bent arm of the bar, and acted on by a 

 micrometer screw. Thus the body is not touched when 

 using the fine adjustment, and the relative distances of 

 objective, binocular prism and eye-piece remain un- 

 changed. 



O 



The smaller stage of the American Centennial Stand 

 is also provided with screws for accurate centring ; this 



