140 



PROGRESS IN MICROSCOPY 



nomena) shows that a small break between two areas, whose path 

 difference equates //2, exhibits, through faulty focusing, a perfectly 

 black diffraction fringe. Such a fringe is exhibited, in MenzeFs plate, 

 by discontinuities, such as A, B, C, etc. A series of fine dark fringes 

 (highly convenient for observation), are thus perceived on the object. 

 Figure 4.16 shows a photograph obtained by this ingenious layout. 



Dark ground 



Surface unevennesses may also be detected by the dark ground. 



Figures 4.17, 4.18, 4.19 show the layout of several devices. Lieber- 



kunn's arrangement is only suited to low magnifying powers. The 



^*— ^ 



yjj 



r^ 



Fig. 4.17. Lieberkuhn device. 



Chapman and Alldridge device (Fig. 4.18) and Reichert's (Fig. 4.19) 

 are catoptric devices. Leitz's ultropak (Fig. 4.20) employs a dioptric 



Fig. 4.18. Chapman and Alldridge device. 



layout. In these instruments the incident light does not pass through 

 the objective. If the object P is perfectly flat, the incident rays are not 

 deflected from the paths shown in Figs. 4.17, 4.18. 4.19 and 4.20. 



