ELEMENTARY PHOTO-MICROGRAPHY. 59 



the iris diaphragm is closed a little ; not too 

 much, or diffraction effects will result, the aim 

 being to increase contrast by securing dark outlines 

 on a light screen. 



Naturally the condenser is the proper place 

 for reducing the angle of the cone of light, but 

 both the Davis shutter and the iris diaphragm will 

 be useful for getting contrast of the object with its 

 background. When this is secured, the dark slide 

 may be inserted and an exposure made. 



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 



