148 PHOTOMICROGRAPHY 



la. Eefocus Substage, Condenser for Parallel Light. This 

 is done by shutting down the iris of the auxiliary con- 

 denser, if it has one, or by covering it with an opaque disc 

 having a small central aperture, or by holding a pencil- 

 point in front of it. Focus the image of either of these 

 on the ground glass by racking the substage condenser 

 up or down. 



8a. Centre the Auxiliary Condenser. Centre the image 

 of its iris by means of the centring screws attached, or 

 move the condenser and lamp together until the image 

 of the iris in the object plane is centred. A good method 

 of examining the collimation of the whole apparatus is 

 afforded by a cap with a very small central aperture 

 fitting the microscope in place of the eyepiece. When 

 the substage iris and auxiliary iris are closed to their 

 smallest apertures light should pass uninterruptedly 

 through the three pin-holes, but any error in centration 

 will prevent that. 



Auxiliary Iris. It is of the greatest importance that 

 the entire aperture of the substage condenser be covered 

 with an even white disc of light if its full aperture is to be 

 used. To make sure that this is so, a card is held in front of 

 the substage condenser : a shadow in the centre of the disc 

 shows that the lamp and condenser require adjustment. 

 Any colour fringes must lie entirely beyond the aperture 

 and should be cut off by means of the auxiliary diaphragm. 



Cooling trough and colour screens are then placed 

 between the condenser and microscope, and focussing 

 and adjustment of the substage iris are carefully carried 

 out as before, and the exposure made. 



A more intense illumination, but still furnished by a 

 parallel beam of light, may be obtained in the way shown 

 in Fig. 47, which is a diagram of Beck's Optical Bench 

 and Imperial Microscope. An image of the radiant in 

 the lantern is projected on the bench diaphragm placed 

 at the focus of a small aplanatic condenser, which directs 



