ADJUSTMENT OF ILLUMINATION 145 



field without an auxiliary condenser, followed by a de- 

 scription of the adjustments necessary when an auxiliary 

 condenser is used to give a parallel or convergent beam. 

 Under proper conditions any of these methods of illu- 

 mination can be used, the choice often depending on the 

 apparatus available. The principles underlying them 

 have already been explained, and the processes them- 

 selves described, so that it is only necessary to give the 

 complete scheme in outline. 



The microscope is placed on the optical bench, and 

 connected with the camera. The lamp is placed as 

 accurately as possible in the optical axis. A white card 

 used in one of the ways already described will be found 

 preferable to the ground glass for adjustment of the 

 illumination, and the camera may if preferred be re- 

 moved while the adjustments 4- to 6 are carried out. 



ILLUMINATION WITHOUT AN AUXILIARY CONDENSER 



1. Centre the Microscope. Fit a low-power objective, 

 but no eyepiece, and see that the disc of light appears in 

 the centre of the focussing screen. 



2. To Centre the Light. Remove the objective, con- 

 denser, and focussing screen, look through the back of the 

 camera down the microscope tube. Move the illuminant 

 until it appears in the centre of the tube. With vertical 

 camera move the mirror until the light appears central. 



3. Centre the Substage Condenser. Screw a low-power 

 objective on to the microscope, place in a low eyepiece, or 

 dispense with an eyepiece altogether ; close the iris 

 diaphragm to a pin-point and focus its small aperture 

 on the ground glass by racking the microscope tube up 

 or down. If the image of the diaphragm is not central, 

 adjust the centring screws until this is so. 



4. Adjust Magnification. Place the object on the 

 stage, attach the objective and ocular it is decided to use. 



