8 PKACTICAL PHOTO-MICROGEAPHY 



sub-stage apparatus and the means for adjusting it are hardly less 

 important. Any instrument for photo-micrography must have 

 a sub-stage condenser, and, as will be found later, the proper 

 use of this is only second in importance to the management of 

 the objectives themselves. The arrangement for carrying a 

 condenser should be thoroughly stiff and solid. Usually it is 

 supported on an arm which extends in a direction at right angles 

 to the stage on its under side. The condenser is carried in a 

 fitting which has freedom of movement in the direction of the 

 optical axis and which is actuated by rackwork for exact focus- 

 sing and adjustment. It, too, should be thoroughly tested to 

 make sure that the movement is a regular one, without lateral 

 displacement, and that the rackwork has no backlash or stiff- 

 ness in action. In some stands a fine adjustment is also 

 provided, exactly as used for focussing the objective ; but the 

 writer has never felt the necessity for this, and regards it as an 

 unnecessary complication at least for any but the most critical 

 and unusual work. A good rackwork motion, used with care, 

 will fulfil every ordinary requirement. The condenser should 

 be that known as the ' centreing ' type that is, it should have 

 an arrangement for altering its position in a direction at right 

 angles to the optical axis, so as to get it in perfect alignment 

 with the axis of the instrument. This is usually effected 

 (compare Fig. 1) by having two screws which engage the mount 

 carrying the optical portion of the condenser. On the side 

 opposite to that on which the screws are situated is a spring 

 which keeps the mount always up to its position. The inner 

 ring carries the optical portion and is movable, by turning 

 either or both screws, in relation to the fixed collars above 

 and below. Underneath the condenser an iris-diaphragm is 

 mounted so that the available aperture of the condenser 

 system may be altered at will. In some Continental stands 

 there is an arrangement for throwing the condenser out 

 of the optical axis of the microscope, so that oblique 

 illumination may be obtained, and, further, there is usually 

 a swing-out arm carrying the condenser which enables the 

 whole of the optical part of the sub-stage to be thrown out 

 of action. 



For metallurgical work a rotating mechanical stage will be 

 found almost essential, as it is necessary to be able to vary 



