REPORT ON PHYSICAL OPTICS. 343 



any distinct idea of the principle of interference itself; and his 

 conception of the mode in which the colours resulted from this 

 " duplicated pulse " is entirely erroneous. Euler was the next 

 who attempted to connect the phenomena of thin plates with 

 the wave-theory of light ; but the attempt, like all the physical 

 speculations of this great mathematician, was signally unsuc- 

 cessful. Euler thought, in fact, that the colours of thin plates, 

 as well as those of natural bodies, arose from emitted, and not 

 from i-eflected light. The incident light was supposed to excite 

 the vibrations of the plate, the frequency of which depended on 

 its thickness, in the same manner as the frequency of the vibra- 

 tions of the column of air in a tube depends on its length. These 

 vibrations again were believed to excite those of the luminiferous 

 ether, and thus to produce the sensation of various colours, the 

 red corresponding to the less frequent vibrations, and the violet 

 to the most frequent*. 



The subject remained in this unsatisfactory state until the 

 principle of interference was discovered by Young. When this 

 principle was combined with the suggestion of Hooke, the whole 

 mystery vanished. The application was made by Young him- 

 self, and all the principal laws of the reflected rings were readily 

 and simply explained by the interference of the two portions of 

 light which are I'e fleeted at the two surfaces of the plate f. In ap- 

 plying this principle, however. Young perceived that the interval 

 of retardation was not simply that due to the difference of the 

 paths traversed by the two pencils ; but that one of them must 

 be supposed to undergo a change of pliase, amounting to half 

 an undulation, at the instant of reflexion. Young clearly pointed 

 out tlie accordance of this eff"ect with mechanical principles ; 

 and the connexion has been fully confirmed hy the more com- 

 plete investigations of Fresnel. In fact, the two reflexions take 

 place under opposite circumstances, one of the portions being re- 

 flected at the surface of a rarer, and the other at that of a denser 

 medium ; and the laws of impact of elastic bodies indicate that 

 the direction of the vibratory movement must be reversed by 

 i-eflexion in the one case, while in the other it is unchanged. 

 Young had the satisfaction of putting this principle to the test 

 in a remarkable manner. It followed from it that if the thin 

 plate were of a refractive density intermediate to those of the 

 two media within which it was inclosed, the laws of the phe- 

 nomenon would be determined by the difference of the paths 

 alone, the reflexion being of the same kind at the two surfaces. 



* Mem. Acad. Berlin, 1752. 



•|- " On the Theory of Light and Colours," Phil. Tnms. 1802. 



