276 



Transactions of the Society. 



inside the tube (E, fig. 52), to which it is attached at its lower end. 

 Since the upward pull of the spring and the downward pressure of 

 the focussing screw are not in the same line, there results a couple, 

 tending to rotate the whole substage and tube in a vertical plane 



Fig. 52. — Part Section through Lower Part of Lime, showing 

 Substage, viewed from Front. 



11, fine-adjustment screw; S, bracket of substage; T, focussing screw of 

 substage ; U, rod which prevents rotation of E ; V, spring of substage. 

 Other letters as in figs. 49, 50, 51. 



about the lower end of the focussing screw. Advantage is taken of 

 this couple to hold the tube against its geometric guides, the upper 

 and lower pairs of guides being placed on opposite sides of the tube. 

 The upper pair of guides is formed by two adjustable screws (one 

 shown at F, figs. 52, 53), which serve to procure perfect alignment of 

 the substage slide with the slides of the body-tube. By this device 

 it is possible to secure the alignment of the body-tube optically 

 instead of mechanically, so that far greater accuracy is obtainable. 



As has already been pointed out, rotation of the guide-tube of 

 the substage about its long axis is prevented by means of a rod, 

 which passes through a hole in the stage. This rod makes no 

 attempt to fit in the hole through which it passes, but presses on 

 one side of it only. The rod is prevented from leaving this side of 

 the hole by the device of winding up the long spring (V, fig. 52), 

 which lies inside the guide-tube. This spring has, consequently, a 

 tendency to rotate the substage about the long axis of the tube, in 

 such a direction as to hold the rod against the side of the hole. 



The long spiral spring, enclosed within the guide-tube, is thus 

 seen to be the key to the whole substage mechanism. It causes the 

 substage to follow its focussing screw without backlash, holds the 

 tube against its four guides, prevents rotation of the tube about its 

 long axis, and allows of the alignment of the substage slide with 

 the body-tubes. 



