66 



PRACTICAL MICROSCOPY. 



The use of the condenser may be demonstrated in 

 various ways, but when employed in connection with the 

 diaphragm, one of its uses may be shown by taking a slide 



FIG. 42. 



of G. angulatum, and using the condenser with a small 

 central aperture. In this way it will be found very difficult 

 indeed to resolve the markings properly, but if the central 



light be stopped out, so as 

 to utilise the oblique rays 

 only, the same effect will be 

 produced as by swinging 

 the mirror round to the 

 side and getting obliquity 

 in that way. 



In using the achromatic 

 condenser, great care must 

 be taken to insure the co- 

 incidence of the optic axis with that of the microscope, 

 by manipulation of the two adjusting screws. This is 

 most easily accomplished by the use of the centering 



FIG. 43. 



