84 



HAMILTON HARTRIDGE ON 



correction of tlie condenser, and also reduces the effective " tube 

 length" ; for instead of the source being placed at 6 in. approxim- 

 ately in order that spherical aberration may be absent, it has now 

 to be placed at l^ in. only. At this position is placed an iris 

 diaphragm which is mounted in a centring collar, the ring of 

 which is attached to the dove-tail slide in place of the mirror 

 (see diagram 2). Below this iris is a slip of flashed opal glass 



Diagram 2 



Arrangement of Substage for Method 4. 



_A = Slide. E = Iris of illuminant. 



B = Immersion condenser. F = Opal glass disc. 



C = Iris of condenser. G = Electric lamp. 



D = Auxiliary lens. H = Switch. 



which is illuminated from behind by a 4-c.p. 4- volt Osram lamp. 

 The light-source is therefore attached to the microscope itself, 

 an arrangement which has the following advantages : 



(1) The condenser being an oil-immersion lens system, " tube 

 length " is kept constantly at the best setting. 



(2) The light-source is always central with the optical axis of 

 the microscope. 



(3) Tilting or moving the microscope does not upset the 

 adjustments. 



