510 Prof. Arthur Schuster on 



spectrum, and that there is no room for any such property 

 as " regularity " independently from that implied in the 

 connexion between energy and wave-length. A spectrum 

 which possesses the same energy for all wave-lengths w r ould 

 prove complete irregularity of the source, while one in which 

 the energy vanishes except for one definite wave-length would 

 be due to a completely regular source. I confess that on first 

 reading through the investigations which I have quoted, some 

 difficulties presented themselves which it seemed necessary 

 to clear up before these conclusions could be accepted in their 

 full generality. There could be no doubt that Gouy's and Ray- 

 leigh's results were correct in the case specially examined by 

 them, in which the analysing spectroscope has a resolving power 

 which is gradually increased ; but if w r e go to the other limit 

 and gradually diminish the resolving power, would it neces- 

 sarily follow^ that all sources of white light behave in the same 

 way, and that the bright and dark bands would in every 

 case disappear at the same time, the difference of path being 

 kept constant ? A test case (7) occurred to me, which seemed 

 to leave a loophole through which we might still save some 

 of our inherited notions about " regularity " of vibration. 

 Although further investigation proved that the difficulties 

 were only imaginary, and that we must accept the new views 

 without restriction, I think that the importance of the subject 

 deserves a more detailed treatment than has yet been given 

 to it. 



In order to put the question at issue as clearly as possible, 

 it is necessary shortly to allude to the history of the subject. 

 2. The fact that rays of light emanating from two different 

 sources do not produce so-called interference phenomena is 

 fundamental, and was always recognized as such. It natu- 

 rally suggested the idea that the vibrations of luminous 

 particles are frequently disturbed, and this, again, led to the 

 further conclusion that interference between rays emanating 

 from the same source should cease if the difference in path 

 were sufficiently increased. Fresnel looked on each particle 

 of matter as vibrating during a certain time according to the 

 simple pendulum law, and considered the group of waves sent 

 out by the particle. If the group is divided into two, one 

 being retarded as compared with the other, " interference " 

 will take place. If the difference in path is equal or greater 

 than the length of the group, Fresnel argued that interference 

 should cease. " But," he adds, "another cause much sooner 

 prevents our observing the mutual interference of two systems 

 of waves, when the difference of path is somewhat great : it 

 is the, impossibility of rendering light sufficiently homo- 



