58 



Modern Microscopy. 



the other, either two or all three can be rotated together or in 

 opposite directions to one another, and the effect is most 



striking. This is an expensive 

 item, but a modification of it is 

 made by Swift and Sons, called 

 the mica-selenite stage, as shown 

 in Fig. 19. This consists of a film 

 of mica made to rotate in a brass 

 plate, upon which the object is 

 laid, and beneath it is a carrier 

 with three separate selenites. 

 These can each be pushed separ- 

 ately beneath the mica and the 



latter rotated. By this means all the different tints obtain- 

 able with any number of selenite films can be produced. 

 It can be employed on any microscope. To get greater 



FIG. 18. Barker's Selenites. 



FIG. 19. Mica-selenite Stage. 



brilliancy the polarizer can be made to fit into the con- 

 denser on the under side, and the Abbe illuminator referred 

 to previously on p. 52 is particularly suitable for this 

 arrangement. 



The Bull's-eye Condenser. 



Many objects, being opaque, cannot be viewed by light 

 from beneath, and consequently have to be illuminated 

 from above. In order to do this a bull's-eye condenser is 

 necessary. These are generally mounted on a stand, and 

 may be had in varying sizes. They consist usually of a 

 plano-convex lens mounted, as shown in the figure. This 



