46 PHOTOMICROGRAPHY 



mechanism for the substage, and its smaller adapt- 

 ability to a wide range of condensers, but centring 

 screws are now added to the condenser rings of a few 

 stands of this pattern, or to the condenser mount itself, 

 or, as an alternative, to the nose-piece. 



The diaphragm with rack -and -pinion movements 

 permits of a more complete control over the illumination 

 than is usually obtained by the English substage, but it 

 increases the complication of the fittings, and the move- 

 ment is not often required. 



On the whole, the writers prefer the English pattern, 

 with its greater stability and centring substage arrange- 

 ments. These features are strong points in the design 

 of the " Premier " microscope, which also includes the 

 more complete illuminating device of the rackwork 

 substage diaphragm, so combining the advantages of 

 both types. 



