648 



DARK GROUND ILLUMINATION 



ICH. XV 



(3) Diaphragms to shut off the direct light of 

 the lantern. These may be simply sheets of tin. 



(4) Glass cells with parallel faces. 



914. Method. Light from the arc L, is ren- 

 dered nearly parallel by the lantern condenser Ci. 

 The diaphragm Di, cuts off the lower half of this 

 beam, the other half serving to illuminate the speci- 

 men S, in the glass cell. The distance between the 

 condenser and the specimen should be from 50 to 

 100 cm. (2 to 4 feet). Either before or after pas- 

 sing through the specimen S, (preferably before, as 

 in fig. 375) this light strikes the special condenser 

 Cz, which brings the diaphragm Di, to a focus at D*. 

 At this point is placed the diaphragm D 2 , which is so 

 arranged as to just cut off the remainder of the light, 

 its edge coming to the edge of the image of the dia- 

 phragm DI. The objective O, is focused to bring 

 the specimen S, to a sharp focus on the screen before 

 the diaphragm D 2 , is in place. With the apparatus 

 thus arranged the screen will be perfectly dark, all 

 light not intercepted by the first diaphragm being 

 stopped by the second. If, now, the liquid in the 

 cell S, is not quite homogeneous but is cordy, as 

 when glycerine and water are first mixed or when a 

 crystal of salt is dissolving, the image of DI, will not 



FIG. 375. DARK GROUND ILLUMINATION; TOEPLER METHOD 

 OF STRIAE. 



L Arc. 



C, First part of the magic lantern condenser. 



D l 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. 



D 2 Diaphragm intercepting direct light from the lantern. 



O Objective. 



a 1 Image of a. 



Note that with the objective on the axis only the upper por- 

 tion of the objective is used. 



