ELEMENTARY PHOTO-MICROGRAPHY. 31 



the velvet connecting tube over the eyepiece. The 

 sliding block is secured by the thumbscrews, and 

 the figure on the ground-glass at the end of the 

 camera is examined. If the magnification be too 

 small the camera is racked out, or if too* big closed. 

 The object is centred by means of the mechanical 

 stage. If size and position are correct fine focus- 

 sing may be performed with the glass through the 

 transparent places at each corner and in the centre 

 of the screen. 



Perhaps the object will be too brilliantly lighted, 

 and the whole be drowned in a flood of light, If 

 so 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. Either the Davis shutter or the iris 

 diaphragm will give contrast. When this is secured 

 the dark slide may be inserted, and an exposure 

 made. 



Objects such as polycystina and diatoms show 

 much better as light figures on a black ground. To 

 obtain this effect the substage condenser and dia- 

 phragm plate will be needed. Let them therefore 

 be attached and the microscope focussed on the 

 object as before. The lamp is turned quite round, 

 keeping the edge of the flame at the same distance 

 from the object as it will be when reversed, and 

 the edge of the flame focussed on the object. This 

 is termed " critical light," the best possible illumina- 

 tion for examination purposes, though too small in 

 area for low-power photography. We must there- 



