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



depends on the amoiint of magnificatiou of the lens , so tbat except 

 in very special cases, the ring-s form but an imperceptible and fading 

 off edge of the illuminated surface. 



All tbe effects we bave as yet considered are those whicb 

 occur in tbe plane in whicb tbe conjugate focus of tbe luminous 

 souree lies. If we take auy plane before the ligbt comes to a focus 

 {K fig. 2), er a plane furtber on tban tbe focus (L fig. 2), the amount 

 of ligbt concentrated at and near P' is spread over a larger sur- 

 face, whicb is tberefore less bright point for point. Similar reasoning 



to tbat bitberto, shows tbat we get an 

 evenly illuminated disc witb fading off 

 edges. 



Next we will consider tbe effect of 

 colour, and see what bappens wben tbe 

 centre of disturbance emits white ligbt. 

 We bave , in tracing tbe patb of 

 tbe wave front in figure 2 , observed 

 tbat it bas on emergence from tbe lens 

 assumed a spberical sbape converging 

 towards P', as a result of tbe velocity 

 of tbe ligbt being slower in tbe glass 

 tban in tbe air. Now wbilst ligbt of 

 all colours travels witb equal velocity 

 in air, they travel witb different velo- 

 cities in glass or otber dense media 

 — tbe sborter tbe wave lengtb of tbe 

 10. Iiglif7 tbe less tbe velocity. As a result 



of tbis ; tbe wave front on emergence 

 from the lens converges to different points for ligbt of differing wave 

 The sborter tbe wave lengtb , tbe nearer tbe point of 

 Figure 10 shows tbis. Tbe dotted lines converging 

 towards P'' represent violet ligbt wbose wave lengtb is from | to |- 

 that of tbe red ligbt represented by tbe otber lines converging 

 towards P''. In tbe diagram in order to obtain a markecl difference 

 of foci, their respective velocities in tbe glass are sbown as 6 to 8, 

 but actually 14 to 15 would be nearer tbe mark. 



Selecting orange as being visually tbe most luminous colour 

 and tbe one we unconsciously focus for, it follows tbat in tbe plane 

 in whicb tbis is brougbt to a focus , tbe violet ligbt bas already 

 passed its focus, wbilst the red bas not yet come to a focus. Conse- 



lengtbs. 



convergence 



