14 Rheinberg: Common Basis oftheTheories ofMicroscopic Vision. XIX, 1. 



As the liglit of auy one wave length does not prodiice perceptible 

 interference etfects with light of another wave leugth in the i-ange 

 of the Visual spectrum, we may represent the whole series of discs 

 and rings of various wave lengths as being superposed as indieated 

 in figure 11. The result is that the composite disc has a white 

 centre with browuish red edges, and the rings have inner violet and 

 outer reddish edges. When a number of discs of monochromatie 

 light overlap , we found that tliey produced an evenly illuminated 

 area with fading off edges, similarly with Avhite light we obtain an 

 evenly illuminated white' area with only its border coloured. 



The colour effect above described, be it again noted, is irre- 

 spective of the freedom of the lens from chromatic aberration, but 



no undue weight must be attached to it, owing 



to the exceeding narrowness of the coloured edge 



as soon as the effective aperture of the lens 



exceeds a few millimetres. 



We now come to the third and last matter 



to be considered in connectiou with the lens, viz. 



s p h e r i c a 1 a b e r r a t i o n. 



If we divide the emergent wave front of a 



lens up into zones, the latter are not all equally 

 *ß curved towards a common point , as they would 



be in the case of a truly spherical surface. 

 ^'^' Instead of this the zones have an increasing 



curvature as we proceed from the central one 

 outwards. As each zone sends forward light in a perpendicular 

 direction to itself, they each have their own focus {F"' to i'^* fig. 12), 

 in i)lace of all the light being concentrated in a Single focus , and 

 this causes the separate points of the source of light to be repre- 

 sented by unduly enlarged discs. The smallest disc would be in 

 the plane of F"^ in the diagram. In this way overlapping occurs, 

 of the Images of poiuts of light a greater distance apart than other- 



^) To arrive at this result, tlie source of light is supposed to be 

 divided up into separate centres <jf disturbance situate at any given small 

 fraction of a wave length from each other. Each colour is considered 

 separately , and the source therefore gets divided into a greater number 

 of centres for colours of short wave length, than for those of louger wave 

 length. This must not be overlooked, for if we considered the source 

 divided into the same number of centres for all colours we should find 

 that we obtained a coloured area instead of a white one. 



