514 Prof. Arthur Schuster on 



any interference experiments distinguish between the light 

 sent out on the one hand by a regularly vibrating system of 

 molecules, and on the other hand by irregular short impulses? 



6. One of the fundamental facts of our subject is the im- 

 possibility of two different sources producing interference 

 phenomena. This fact is generally taken as a proof that 

 molecules must suffer frequent disturbances. But two bodies 

 sending out what we have called regular white light would 

 have the same property. For imagine each of the two 

 sources to send out a series of waves of lengths \ 1? X 2 , &c, 

 differing by quantities which are so small that no instrumental 

 power at our command can resolve them. The illumination 

 at any point of a screen will depend on the manner in which 

 the trains of waves coming from the two sources will combine. 

 The difference of phase between the two sets of waves of length 

 Xj depends not only on the relative distance of the sources 

 from the screen, but also on the difference of phase at the 

 two sources. As there is no connexion between that difference 

 of phase for the different wave-lengths, the wave-lengths in 

 close proximity to \ will partly destroy each other, partly 

 have fourfold or any intermediate intensity, the result being 

 the same at all points of the screen as if no interference had 

 taken place. This leads to an important result. Imagine 

 a body, the molecules of which vibrate in a perfectly definite 

 period, but have a translatory motion such as we imagine the 

 molecules of a gas to possess. Such a body would give a 

 spectrum of one line, which, however, would have a certain 

 width owing to the motion of the molecules. The want of 

 homogeneousness produced by this translatory motion would 

 be quite sufficient to prevent the possibility of interference 

 between two similar sources, and we need not take refuge in 

 the impacts between molecules to explain the non-existence 

 of interference. 



7. In fig. 1 let L x and L 2 be two lenses, H x H 2 and K x K 2 

 two screens in their focal plane having small apertures at the 

 foci G and F. If ACB is a grating, a disturbance passing 

 through G will produce a certain 



effect at F. Unless the relative Fig. 1. 



positions of what for convenience' 



sake we may call collimator and 



telescope are adjusted so that a 



direct image of G is formed at 



F, the optical lengths of rays 



such as GBF, GCF, GAF, will 



not be the same, consequently 



an instantaneous impulse at G k, f i% 



will not remain an instantaneous 



H 2 



