ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



645 



2. A permanently attached combination wedge. — A great deal of time 

 is ordinarily lost iD picking up the accessories to the Microscope and in 

 hunting for the slot into which they are to be inserted. The author has 

 found that the simple contrivance shown in fig. 82 overcomes all this. 

 A carriage, exactly fitting the slot above the objective, is inserted in the 

 tube of the Microscope, and is kept in place by two end screws like those 

 holding the Bertrand lens bar. At one end is a square of gypsum, a, 

 giving red of the first order ; b is an opening ; and c is a quartz wedge 

 underlain by a mica plate, the two minerals having their directions at 

 right angles to each other, and similar in construction to a Wright 



D 



Fig. 83. 



quartz gypsum wedge. The thickness of the underlying mica plate is 

 so chosen that it exactly compensates the front end of the quartz wedge, 

 consequently the colours of the combination wedge begin at darkness 

 and gradually increase to the fourth order as the wedge is pushed forward. 

 A spring S, attached to the side of the Microscope tube, presses against 

 the carriage and produces enough friction to hold it wherever it is placed. 

 When the opening b is centred, the spring drops in a rounded notch as 

 shown. Upon the upper side of the carriage a scale is engraved, and the 

 •end of the spring shows the order of the colour at that time beneath the 

 cross hairs of the Microscope. 



3. A rotating loiver nicol for observing very slight pleochroism. — A 



