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



to iindulations of tlie etiler in wliich tlie individnal etlier particles 

 do not vibrate merely to and fro in a straiglit line, but raay exe- 

 cute movements in any of tbe numerous shapes of an ellipse (fig. 1 a), 

 we need not bere complicate matters by tbese considerations, since 

 tbe amount of disphicement to and fro in a certain direction remains 

 tbe same no matter wbat tbe sbape of tbe ellipse, seeing tbat tbe 

 particles move witb simple barmonic motions. 



1. 



la. 



We can tbns ascertain tbe liglit dne at any point, from one 

 Single centre of disturbance , by compounding tbe pbases at Avbicb 

 it arrives tbere after travelUng tbere by different patbs. 



Not so bowever witb unduhitions coming from any two of tbe 

 innumerable points of wbicb a luminous source is mostly constitnted. 

 From tbese, tbe trains of waves start qiiite independently of one 

 anotber at irregulär intervals wbicli bear no relationsbip to one 

 anotber, Conseqnently undiilations coming from sncb different centres 

 of disturbance, instead of creating a regulär difterenee of intensity 

 tbrougbout tbe wbole period of time, tbey create a constantly cbanging 

 intensity. Tbe clianges occur iufinitely too frequently for tbe eye 

 to observe, and it tberefore comes about tbat tbe observable eft'ect 



