XIX, 1. Rheinberg: Common Basis ofthe Theories ofMicroscopic Vision. 9 



R and T^ and gradually moiinting up to something less than ^/.^.^ of 

 the light of P' at its brightest point S (fig. 7). 



By proceeding as before, we conld show that there are further 

 consecutive dark and light rings surrounding the central diso, bnt 

 tliey are so exceedingly faint as to be a negligeable quantity. 



7. 



Fignre 7 mnst not be considered otber than a diagram to give 

 a general idea ilhistrating the expbination given. The way in which 

 exact distribution of intensity over the curve and rings is obtained, 

 cannot be qnite so simply shown by diagrams , for we really liave 

 to deal with the intersection of splierical surfaces 

 (fig. 8) , whilst the diagrams only refcr to the phase Q- 

 of their median sections X Y and JC Y' . But this 

 only alfects the result in altering the shape of the 

 intensity curve somewliat. It would seeni a prion 

 that there was a perfectly gradual trausition from 

 light to darkness and vice versa, from P to R, R 

 to *§, S to T etc. whilst from experimental evidence 

 the transition is more or less abrupt (fig. 9) the bright 

 rings being separated from the disc by a more or 

 less broad ring of perfect darkness. The discs and 

 rings themselves are also of approximately uniform intensity except 

 at their edges, there are however otlier reasons to account for this 

 as we shall see a little further on. 



We have not yet taken any note of the distance of the rings 

 from the disc , but from the diagrams 3 to 5 it must be apparent 



8. 



