CH. XV] 



DARK GROUND ILLUMINATION 



649 



be quite sharp and some light will escape the edge 

 of the diaphragm D 2 , and reach the screen. 



The result is very striking as even a slight inhomo- 

 geneity of the medium in the glass cell will deviate 

 light sufficiently to pass the second diaphragm and 

 thus be seen. 



Suppose a slight cord of a substance of different 

 refractive index from its surroundings to exist at a 1 

 in fig. 376. This cord will scatter the light. A ray 

 which would normally strike D 2 , and be intercep- 

 ted, will spread out in all directions. The part of 

 this light which strikes the objective will go to form 

 a screen image of the cord at a 1 (fig. 376). 



The sensitiveness of this method depends upon the 

 sharpness of the image of the diaphragm DI, and the 

 closeness of adjustment of D 2 , so as to encroach as 

 little as possible upon it. With a very sharp im- 

 age, it is possible to detect the minutest striae and 

 inhomogeneities in the specimen. 



The image sharpness may be disturbed as much by 

 imperfections in the condensing lens C 2 , as by an in- 

 homogeneity of the specimen, hence these imperfec- 

 tions, if present, will show distinctly on the screen. 

 In fact, the method is as well designed to show the 



FIG. 376. 



DARK GROUND ILLUMINATION; TOEPLER 

 METHOD OF STRIAE. 



L Arc. 



C t First part of the magic lantern condenser. 



>! Diaphragm. 



C 2 Condenser of long focus. It must be as perfect a lens 

 as can be found. 



S Specimen with slight inhomogeneity. 



a An inhomogeneity in the specimen which deviates the 

 light. 



DI Diaphragm intercepting direct light from the lantern. 



Objective. 



a 1 Image of a. Note that a small objective above the axis 

 is used. The dotted lines show the course of the rays which 

 are slightly deviated from their original path by the inhomo- 

 geneity of the specimen. 



