is perforated with a series of openings of different sizes, 

 each of which may be brought into the optical axis. 



Besides this there are other forms which may be 

 said to be better for instance the so-called cap dia- 

 phragms, which require a separate piece for each 

 aperture and which are held by a special substage 

 receiver. There are usually three cap diaphragms, 

 each one having an aperture of different size, and 

 when attached are located below and near the object. 

 While the results obtained are much better than with 

 a revolving diaphragm, a change of diaphragm is 

 inconvenient as it involves the removal of the slide 

 from the stage or the receiver from below the stage. 

 An improved form has of late years been applied to 

 the better instruments in the iris diaphragm, which 

 consists of a series of thin overlapping blades placed 

 around a central opening the size of which may be 

 varied by means of a lever or milled edge operating 

 the blades. Besides the possibilities of varying the 

 size of the aperture there should be an adjustment for 

 changing the distance from the object. The distance 

 of the diaphragm from the object is one of considerable 

 importance. The best position is just below the sur- 

 face of the stage, but as this is not always possible, it 

 should be as near as conditions will permit. Very 

 recently it has been possible so to construct the iris 

 diaphragm, that it passes up through the opening of the 

 stage and may thus be brought very close to the object. 



54 



