174 PKACTICAL PHOTO-MICKOGKAPHY 



that its image just encroaches on the field of view. By this 

 means any extraneous stray light or reflections are considerably 

 reduced, and diffraction images from portions of the object not 

 in the field of view do not have a disturbing effect. By attend- 

 ing to this point carefully the purity of the resulting image is 

 appreciably increased. 



In the cases already described where one collecting-lens 

 is placed near the source of light, and is allowed to act as a 

 collimator, throwing a parallel beam on to a second condensing- 

 lens, it can be observed whether a parallel beam is being pro- 

 jected in the right way by placing a piece of white paper in 

 the path of the rays. The illuminated area should then be 

 quite even, with no darkened areas or bright spots in any 

 part of it. Practically, of course, it is impossible to obtain a 

 perfectly parallel beam of light, but a sufficient approximation 

 to this may easily be secured. 



To summarise, when using a single collecting-lens, of 

 whatever type, the image of the lens-aperture must be pro- 

 jected in the plane of the object, or alternatively the aperture 

 of a diaphragm placed close to the lens may be so focussed. 

 With a compound system of collecting-lenses a diaphragm 

 may be placed in a position where a parallel beam is being 

 projected and an image of the diaphragm-aperture thrown on 

 to the object by the sub-stage condenser. In either case 

 steps must be taken to ascertain that the full working aperture 

 of the sub-stage condenser is being utilised. 



Any collecting-lens must be placed exactly in line with 

 the optical axis of the microscope and with its face at right 

 angles to the optical axis. The latter adjustment is best 

 secured by observing the faint image that is reflected from each 

 lens surface. The reflected image should return in the same 

 direction as the direct beam, and can usually be seen as a 

 more or less central disc of light on any diaphragm in the 

 path of the rays. 



