650 



DARK GROUND ILLUMINATION 



[Cn. XV 



imperfections of the second condenser C 2 , as to show 

 the specimen. The difficulty can, of course, be les- 

 sened by drawing the condenser face C 2 , out of the focus 

 of the objective O. Dust, fingermarks, etc., will then 

 produce a general blur, rather than a distinct image. 



915. Foucault's method. A slight modification 

 of the old method of Foucault for testing telescope ob~ 

 jectives also gives good results. This method (fig. 377) 

 dispenses with the use of the first diaphragm Di, the 

 crater of the arc being the first diaphragm in this case. 

 Instead of the ordinary condenser, is substituted a lens 

 or set of lenses which are to form a very sharp image of 

 the crater. Just in front or behind the objective 

 (wherever the sharp image of the crater is formed) is a 

 diaphragm which just covers up the crater image. Such 

 a diaphragm may be made by fastening a round 

 piece of black paper to a piece of plate glass. The ob- 

 jective brings the specimen to a focus upon the screen 

 in the usual way. Any inhomogeneities in the speci- 

 men scatter the light so that some of it gets by the 

 central stop at D 2 . It is this scattered light which 

 serves to form the image on the screen at a 1 . 



If the inhomogeneities of the specimen are great 

 enough, the specimen may be projected by the method 

 just described, (see fig. 377), except that instead of 

 using a central stop, the lens is provided with an iris 

 diaphragm, and the image of the arc is focused on 



L 



C 

 S 

 a 

 O 



I), 



F IG - 377- DARK GROUND ILLUMINATION, 

 FOUCAULT'S METHOD. 



Arc. 



Condenser of long focus, as perfect as can be found. 

 Specimen. 



Inhomogeneity in the specimen which deviates the light. 

 Objective. 



Central stop to intercept the direct light from the arc. 

 a 1 Image of a, projected on the screen. 

 The dotted lines show the course of the rays deviated by an 

 inhomogeneity in the specimen which pass to one side of the cen- 

 tral stop and reach the screen. 



