THE SUB-STAGE 



I8 5 



The vertical and lateral movements need not be as elaborate as 

 those of the stage, since only a small movement in each direction is 

 required. The object is to secure a centring motion, a motion that 

 will make the optical axis of the sub-stage combinations continuous 

 with the optical axis of the objective. It must therefore be a steady 

 motion ; the sub-stage must move decisively, and must rigidly re- 

 main in the position in which it is left. 



A bad sub-stage moves in jerks, and is liable to spring from the 

 position intended to be final. 



It is not needful that the motion should be in right lines ; 

 motion in arcs whose tangents intersect at right angles are quite as 

 efficient. A steady, even, reliable motion that will enable a centre 

 to befoimd is all that is required. 



FIG. 147. Beck's mechanical attachable stage (1896). 



The focussing adjustment must be smooth, steady, and firm, acting 

 readily and remaining rigid. The recent employment of achromatic 

 condensers of wide apertures has led such critical workers as 

 Mr. E. M. Nelson to suggest a fine adjustment to the sub-stage. 

 There are times when it is a great luxury and a facile path to 

 delicate and desirable results ; but it may be quite simple, a direct- 

 action screw of fine thread, or a cone which the revolution of 

 a screw pushes horizontally forward upon the bottom of a sliding 

 bar to which the sub-stage is fixed, or an inclined plane acting 

 in a slot in the same way. In fact, any simple device for focussing 

 the condenser more slowly than the rackwork will do, pushing the 

 condenser up to, or causing it to recede from, the under surface of 

 the slide with sufficient delicacy. But no means should be employed 



