542 Prof. Arthur Schuster on 



irregular, but the medium may take up and propagate some 

 vibrations quicker than others. 



There are many signs tending to show that the time is not 

 far distant when, in order to explain the connexion between 

 optical and electrical facts, we must recognize some structural 

 properties of the medium, and the regularity in the vibration 

 of a black body may be intimately connected with such 

 structural properties. 



21. The result of this paper is in complete agreement 

 with the conclusions formed by Grouy and Lord Rayleigh as 

 to the nature of white light. We have obtained an expres- 

 sion (§ 8) for the disturbance produced by a grating if 

 the disturbance in the incident light is given, and traced the 

 result in special cases. We have more especially examined 

 the case in which bright and dark bands are formed in a 

 spectroscope when two sets of rays are brought to inter- 

 ference, one having a certain retardation compared to the 

 other, and shown (§14) that provided the intensity of light 

 does not sensibly vary from one band to the next, the relative 

 intensity of the maxima and minima can be given in terms of 

 the retardation and resolving power without any hypothesis 

 as to the nature of light. So conclusions, therefore, can be 

 drawn as to the " regularity " of white light. The nature of 

 light, including its " regularity/' as follows from Rayleigh's 

 investigations, is completely denned by the distribution of 

 energy in the spectrum. 



The analysis of the spectrum by a prism is shown (§ 15) to 

 involve, like that of a grating, the spreading out of an impulse 

 over a length of time which increases with the resolving 

 power. That is to say, a single impulse falling on the slit 

 of the collimator will not pass through the focal plane of the 

 telescope as a single impulse, but as an oscillatory motion. 

 This follows in a simple manner if we consider the passage 

 through a prism of a " group " of waves, the front of which 

 moves with the group velocity. 



In § 18 an expression has been obtained for the distribu- 

 tion of light on a screen, in a case of simple interference like 

 that of Lloyd's mirror arrangement. The expression again 

 only involves the distribution of energy in the spectrum. If 

 all wave-lengths were equally represented, there would be no 

 interference at all. The coloured bands which are seen in 

 this case, as well as the colours of thin films, are shown, § 20, 

 to be due to interference produced in the eye itself, and to 

 depend on the fact that in any substance showing selective 

 absorption a single impulse produces an oscillatory motion 

 lasting for some time. In order that two impulses should 



