THE MICROSCOPE BY MEANS OF THEIR OPTICAL CHARACTERS. 599 



The upper nicol usually slides in and out of the lower part 

 of the tube above the objective. This has the advantage of 

 not obstructing the field, but there are two objections to this 

 plan. In the first place the nicol cannot, as usually constructed, 

 be rotated, and secondly, it does not allow of the insertion of 

 a quartz wedge in focus. For this reason the upper nicol is 

 sometimes placed above the eye- piece. In this case it should 

 be thrown in or out by means of a hinge. The common 

 arrangement by which it is removed altogether results in loss 

 of time in adjustment when it is replaced. The nicol that 

 rotates should be provided with catches or ""clicks" to arrest 

 its movement in the crossed position, and in that at right 

 angles to it, in which its direction of vibration is parallel to 

 that of the other nicol. 



Cross Wires. The cross wires will be parallel to the directions 

 of vibration of light traversing the nicols respectively when in 

 their normal position. 



The cross wires should not be spider lines, which are easily 

 broken by the insertion of the quartz wedge or other accessories, 

 but should be ruled on a glass plate. As this is apt to get 

 covered with dust, the eye-piece should be made to screw apart 

 immediately above the plate so that it may be easily cleaned. 



Slots. The microscope should be provided with one or more 

 slots for the insertion of various accessories. In this country 

 slots are placed diagonally to the cross wires. On the Continent, 

 however, they are sometimes right and left, and accessories 

 connected with polarization effects, such as quartz wedges, 

 gypsum plate, or mica steps, must be constructed accordingly. 

 This is a matter that requires attention in buying and working 

 with foreign microscopes. The slot is usually placed immediately 

 above the objective. Wright, however, prefers to have it below the 

 stage (but naturally above the lower nicol), so that the insertion 

 of a plate or wedge, like that of the lower nicol referred to above, 

 does not affect the field. Another course is to have the slot 

 at the focus of the eye- piece, in which case the upper nicol must 

 be placed above the eye-piece. This arrangement has the 

 advantage that a quartz wedge, or other accessory, placed in 

 the slot is in focus. The same result can also be obtained 

 with a slot below the stage, if the condenser be placed in position 

 and slightly lowered. 



