70 PRACTICAL MICROSCOPY. 



by a rectangular prism, for condensing light into the achro- 

 matic combination. The next part consists of a second 

 arc-piece placed at right angles to the former one ; this 

 also carries a similar achromatic condenser and illumi- 

 nating prism, which move radial to the centre. Both these 

 arc-pieces are so divided that each pencil of light can be 

 projected at a similar angle, and previous results can 

 always be recorded in the same way. Difficult test objects 

 are readily resolved, especially such diatoms as have rect- 

 angular striae or markings. With a -inch objective, the 

 diatom Navicula rhomboides is easily resolved into squares. 

 The markings on G. angulatum by the same means are 

 made to stand out in bold relief like half spheres. Those 

 usually considered easily resolvable only require one pencil 

 of light to show the markings. When this is the case, the 



FIG. 48. 



rectangular arc-piece with all its illuminating apparatus can 

 be turned away from the microscope stage as shown in 

 Fig. 48. The same illustration shows how opaque objects 

 may be illuminated, viz. by moving the condenser of the 

 first arc-piece above the stage of the microscope, when a 



