1 26 PHOTOMICROGRAPHY 



employed in place of the convergent beam just described. 

 The condenser is focussed just as before, and the following 

 conditions must be complied with. The beam of light 

 must be sufficiently broad to cover the entire back lens 

 of the substage condenser, or such part of its aperture 

 as is made use of, and any colour fringes round the disc 

 of light must fall right beyond the condenser for the reason 

 above mentioned. Such a falling off of light at the edges 

 often occurs when an uncorrected condenser is used, and 

 may effectually prevent the utilisation of the full aperture 

 of an oil-immersion substage condenser. 



Either of the methods described above gives an image 

 that suffers in no respect in comparison with that ob- 

 tained when an image of the radiant is focussed in the 

 object plane, and one or other is almost invariably used 

 for high-power photomicrography, especially as the 

 illumination produced is much more intense than that 

 given by the same radiant without an auxiliary condenser. 



The Auxiliary Iris Diaphragm. The importance of the 

 auxiliary iris is brought out by the above experiments, 

 and it should always form part of the optical bench 

 equipment. When closed down it is used for focussing 

 and centring purposes. It is also used for cutting off the 

 outer zones of the beam projected by the condenser, as 

 only enough light should be used to cover the aperture of 

 the substage condenser, any more is liable to give rise 

 to reflections and mistiness of image. Its aperture 

 is adjusted, after focussing the substage condenser, by 

 examining the image and slowly opening the diaphragm 

 until its edges coincide with the limits of the field of the 

 objective. 



