72 



PKACTICAL PHOTO-MICKOGKAPHY 



ways they can be used with any illuminant, whether of large 

 or small area. For work with high powers, lens I is used 

 singly as a simple collector. It is arranged to project the 

 image of the radiant (assuming the latter to be an arc light or 

 some similar illuminant of small area and high power) upon 

 the diaphragm of the sub-stage condenser of the microscope, 

 magnifying it about four times. In adjusting this it should 



FIG. 21 a, b, c, d. Collecting-lens Systems of Great Light-gathering Power. 



be so placed that the image of the ring surrounding the collect- 

 ing-lens is projected by the sub-stage condenser of the micro- 

 scope into the centre of the field of view. Fig. 21 a shows the 

 arrangement of lenses, &c., set up for high-power work. In this 

 arrangement the image of the radiant should be so large that 

 the entire aperture of the condenser is filled with light, as other- 

 wise its full aperture would not be utilised. But, on the other 

 hand, when using short-focus sub-stage condensers, the image of 

 the radiant projected in the field of view is small, and is there- 

 fore only sufficient for illumination when using high powers. 

 For moderate magnifications lenses I and II are used in 



