184 



THE MICROSCOPE. 



thickness, \/hen the illuminator is brought into immer- 

 sion contact. Its action is diagramatically shown in 

 fig. 117. A is the first lens of an immersive objective 

 in fluid contact with the cover-glass ; o the object in 

 balsam ; p a right-angled prism in immersion contact 

 with the base of the slide ; L a lens designed to focus 



FIG. 117. 



the illuminating rays on the object o. For oblique 

 illumination, as seen in the figure, the apparatus must 

 be thrown out of the axis of the microscope, and in 

 this way and with any objective of less aperture than 

 90 in glass would give a dark field. If brought nearer 

 the axial line it is evident that less oblique rays could 

 be used. 



Mr. John Mayall devised a set of spiral diaphragms 



as a convenient mode 

 of obtaining oblique 

 illumination in con- 

 nection with high-an- 

 gled condensers. If a 

 slot diaphragm, fig. 

 118, be fixed close be- 



Fio. 118. Mayall's Spiral Diaphragm. neath the larger lens, 



such as those of Powell 



.and Lealand, Zeiss and other makers, the rotation 

 under it of a diaphragm having a spiral opening, 



