ELEMENTARY PHOTO-MICROGRAPHY. 49 



object well defined in the centre. The focussing 

 may be altered for this purpose. The eyepiece is 

 replaced to see if the field be still uniformly lighted, 

 a slight adjustment of focus again being necessary. 

 If the lamp be in the right position, the disc of 

 light will be sharp and uniform right up to the edge 

 of the circle with or without an eyepiece. The card- 

 board is taken away, and the camera brought up 

 to the microscope, with the velvet connecting tube 

 over the eyepiece. The sliding block is secured 

 by the thumbscrews, and the figure which will now 

 appear on the ground-glass at the end of 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 focussing glass through 

 the transparent places at each corner and in the 

 centre of 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. 



