Suggestions regarding the Mechanical Design of Microscopes. 137 



designed to slip into the substage ring from above, the diaphragm 

 being nearly always too far from the back lens of the condenser. 

 This is especially noticeable in a large instrument owned by 

 myself, in which, owing to the thickness of the mechanical stage, 

 an adaptor about ^ in. long has to be inserted between the optical 

 portion of the condenser and its mount. 



When one considers the care taken to overcome this difficulty 



/t^ % 



in the early condensers made by Powell, Eoss and others (see 

 Carpenter, 4th edition), it is surprising that this detail is now so 

 neglected. 



It is surely simple to mount the iris in its proper position close • 

 to the back lens of the condenser, and to actuate it by means of a 

 rotating tube within the ring of the condenser mount. The lower 

 end of this rotating tube should project about J in. below the sub- 

 stage ring and be milled. 



