152 PHOTOMICROGRAPHY 



76. Centre and Focus the Condenser. Place the cooling 

 trough next to the condenser, and then the bench dia- 

 phragm. Move the condenser about until the disc pro- 

 jected by the microscope on the ground glass or card is 

 circular and free from colour. Spots or colour fringes 

 on the disc show that the condenser and lamp are not 

 centred. The lamp must be moved until they are, as 

 the condenser is not fitted with centring screws. The 

 iris diaphragm is then closed to a pin-point, and its 

 aperture focussed by moving the substage condenser. 



The remaining operations are as before. 



Use of a Negative Auxiliary Lens. Under some condi- 

 tions it may be found impossible to fill the back lens 

 of an immersion condenser so as to utilise to the full 

 the aperture of homogeneous objectives. In these cir- 

 cumstances the image of the radiant formed on the 

 substage diaphragm may be enlarged by turning the 

 bull's-eye into a sort of telephoto combination by the 

 addition of a concave or negative lens. 



The concave lens (about 6 or 7 in. focus) is placed be- 

 tween the collecting lens and microscope, in position to 

 give an image of the radiant on the back of the iris. It 

 is centred by moving it about until the image of the iris is 

 in the centre of the field, and critical light obtained by a 

 slight movement of the collector towards the radiant to 

 focus the iris again. 



Projection Eyepieces. If one of these is to be employed 

 it is placed on the microscope last of all, and its dia- 

 phragm focussed on the screen by moving the eye-lens. 

 Should any alteration of camera extension be decided 

 upon it must be refocussed. On account of the difficulty 

 of getting exact focus, the use of the tape supplied by 

 Zeiss for measuring the extension and indicating the 

 position of the graduation on the eyepiece for that 

 extension is to be recommended. 



