30 Rheinberg: Common Basis ofthe Theories ofMicroscopic Vision. XIX, 1. 



area , with fading off edges , whicli may or may not include the 

 envelope of some of the overlapping rings or maxima , as ex- 

 plained in chapter IL 



The experiment is easily made. Place a ruling of 500 to 1000 

 per centimetre on the stage, and let the light impinge on it through 

 a narrow slot, placed in the diaphragm holder of the condenser. It 



is immaterial whether the 

 condenser lenses are used or 

 not. Then the ruling will 

 give rise to a central Image 

 of the narrow slot with dif- 

 fraction spectra on each 

 side as previously explained. 

 These may be viewed by 

 focussing a l" objective on 

 the lines, removing the eye- 

 piece and looking down the 

 microscope tiibe. Now gra- 

 dually widen the slot and 

 the consequent overlapping 

 of the spectra from each 

 point of it caiises a cor- 

 responding widening of the 

 central Image and spectra 

 tili at last they overlap and 

 form the even illumination 

 of surface. 



Figure 35 illustrates in 

 another way how the same 

 intensity of light arises in 

 each direction when a slot is illuminated by a converging aplanatic 

 cone of light. Points A, B, C etc. are taken on the grand wave 

 front such that from each consecutive point its wave front would 

 arrive at the two extremities of the slot with half a wave length 

 greater difference than the preceding one, then the light pro- 

 ceeding from Ä gives its chief Image towards äP\ first maximum 

 towards A Q , and first minimum towards A R. The ray from 

 B is the one representing the wave front which arrives at the 

 two ends of the slot with just 4- wave length difterence ; it gives 

 its chief Image at BP'^ its first maximum at B Q and first minimum 



35. 



