116 PHOTOMICROGRAPHY 



the eye and, lamp, and note that it is no longer full of 

 light, but shows characteristic shadow areas on part of 

 its disc. 



3. Keeping the eye and condenser in their first positions, 

 move the lamp out of the axis of the condenser, and notice 

 that shadows appear on the limb of the bright disc. 



4. Place the condenser in such a position that the light 

 is at its focus ; it projects a parallel beam of light, as is 

 evidenced by moving the card to and fro, when it will 

 be noted that the disc remains constant in size. 



5. Move the condenser nearer to the radiant, it pro- 

 jects a divergent beam. 



6. Move it further away, a convergent beam is formed ; 

 and by moving the card into a certain position an image 

 of the radiant is focussed on it. 



7. As the condenser is moved away from the radiant 

 the card has to be moved nearer, and the image on it 

 becomes smaller. 



8. The disc projected by most condensers is not uni- 

 iorm over its entire surface, even when the light is small 

 and placed correctly, but shows a central bright portion 

 surrounded by a broader or narrower ring of shadow or 

 colour. The central portion is all that is useful for illu- 

 mination, and steps should always be taken to ensure that 

 the colour fringes lie entirely outside the surface that has 

 to be illuminated, or are cut off by means of an iris 

 diaphragm. 



9. Compare the disc projected from the same illuminant 

 by corrected and uncorrected condensers of similar size 

 and focal length. The increase in brilliance and the 

 absence of colour fringes that follow the use of the cor- 

 rected lens are noticeable. 



Experiments with the Microscope. These experiments 

 can now be extended to the microscope, which is set up 

 vertically or slightly inclined. A flat -wick oil lamp is the 

 most suitable illuminant. It is placed with the edge of 





