Interference Phenomena. 515 



impulse at F, but the light reflected from each line of the 

 grating will reach F at certain intervals of time, and a 

 disturbance will be set up at F which will last during a finite 

 time. The time it takes for an impulse at G to pass com- 

 pletely through F will be that required by light to go over a 

 space equal to the difference in optical length of the extreme 

 rays GBF and GAF. The fact that in every dispersive 

 system an instantaneous impulse entering the system is 

 changed into a disturbance lasting through a finite time is 

 of fundamental importance. It is easily seen that this is 

 brought about in the case of a grating, but the fact is equally 

 true if the dispersion is due to refraction, as will be pointed 

 out further on. 



Imagine that in fig. 1 two impulses of the same type 

 succeed each other. If the second impulse begins to reach 

 F before the effects of the first impulse have passed away 

 interference may occur, but if the disturbance due to the first 

 impulse has completely passed through F before the second 

 impulse reaches it, no interference can take place, and this 

 will happen whenever the distance between the impulses 

 before they reach G is greater than the difference in the 

 optical length between the extreme rays GAF and GBF. It 

 is thus seen that if light were to consist of separate instanta- 

 neous impulses, interference at the focus of a grating-spectro- 

 scope of finite resolving powers could only take place if the 

 retardation were smaller than a certain amount ; if the retar- 

 dation were greater, no interference could possibly occur. 



Now consider the light to be what we have called regular, 

 that is to say, to consist of the superposition of a large num- 

 ber of homogeneous vibrations. If the beam of light is split 

 into two, one being retarded with reference to the other, the 

 spectrum formed in the focal plane of L 2 will be crossed by 

 bright and dark bands ; if the retardation is gradually in- 

 creased these bands will gradually get nearer and nearer 

 together, and at the same time become less distinct as their 

 distance approaches the limits which are still resolvable by 

 the grating. Looking at it in this general manner, there 

 seems to be no reason why there should be a definite limit 

 when interference ceases, but it would seem as if there ought 

 to be always a fluctuating intensity along the spectrum, 

 although the difference in intensity between the bright and 

 dark bands would with increasing retardation of the inter- 

 fering beams become so small, that they could no longer be 

 traced. This is the test case previously alluded to, by means 

 of which I originally thought we might distinguish between 

 " regular " and irregular white light. It was found, however, 

 that on actually calculating the difference in intensity between 



