50 ELEMENTARY PHOTO-MICROGRAPHY. 



DARK BACKGROUNDS. 



It is possible to light an object by rays thrown 

 at such an acute angle from one or more sides that 

 none of the direct light enters the objective. 

 This is known as dark ground illumination. For 

 examination purposes the light may be reflected 

 from the mirror drawn to one side, but for photo- 

 graphy the object should be lit up from a hollow 

 cone of light, which diverges after passing the 

 focal point on the slide. 



Objects such as polycystina and diatoms show 

 much better as light figures on a black ground. 

 To obtain this effect the substage condenser and 

 diaphragm plate will be needed. Let them, there- 

 fore, be attached, and the microscope focussed on 

 the object as before. Now take away the bull's- 

 eye, and with substage condenser focus the edge 

 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. 



