60 ELEMENTARY PHOTO-MICROGRAPHY. 



of flame on the object, which will give the best 

 possible illumination for examination purposes, 

 though too small in area for low power photography. 

 This is termed " critical light." We must therefore 

 find some means of diffusing it over the whole 

 surface of the object, and this can be done by 

 again interposing the bull's-eye, when an evenly- 

 lit disc on the cardboard is sought as in the method 

 last described. Having got it satisfactorily, the 

 central rays are stopped out by means of a disc or 

 stop in the diaphragm plate, so that an annular 

 ring of light only enters the condenser. 



If no suitable condenser be at hand, a bull's- 

 eye may be requisitioned and made to do good 

 service, especially for large objects. Paste on it a 

 piece of black paper to block out the central rays, 

 and use it in place of a condenser (fig. 18). A 

 spot lens or a Wenham parabolic reflector to fit the 

 substage will also give a dark ground with the 

 object illuminated thereon. 



For living bacteria, etc., several kinds of 

 illuminators in the form of condensers or para- 

 boloids are now in the market for obtaining dark 

 ground with high powers. These are oiled to the 

 under-side of object slide, and direct the light in 

 such a way that no direct rays enter the micro- 

 scope. The object thus appears luminous with 

 dark surroundings. 



