71 



distance of diaphragm from the object is one of 

 considerable importance. The best position is just 

 below the surface of the stage, but as this is not 

 always possible, it should be as near as conditions 

 will permit. The nearer it is, the more intensity 

 will be maintained with suitable moderation by 

 the limit of opening. Very recently it has been 

 possible, to so construct the iris diaphragm, that it 

 passes up through the opening of the stage and is 

 flush with its upper surface. 



