BECK'S VERTICAL ILLUMINATOR. 



217 



thereof traversed by the wheel. By this simple proced- 

 ure a gauge is established from which other covers can 

 be easily measured. Some stands have the wheel of the 

 fine adjustment graduated, which will be found especially 

 convenient. In other cases the observer can make his 

 own graduations on a little circle of paper and adapt 

 the same to the wheel, without any great drain on his 

 mechanical skill. 



BECK'S VERTICAL ILLUMINATOR. 



This instrument consists of a brass tube, either end 

 being fitted with the " society screw," so that it can be 

 attached to the back of the objective; the object-glass 

 with the iluminator thus attached is screwed to the nose 

 piece of the stand. The tube has an adapter of its own, 

 so that it can be revolved around the optical axis of the 



