138 COLOURS OF THIN PLATES. 



on the interval of retardation of the second " pulse," or 

 wave, with respect to the first, and therefore on the thick- 

 ness of the plate. But he does not seem to have had 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 unsuccessful, and the subject remained in this 

 unsettled state, until the principle of interference was dis- 

 covered by Young. When this principle was combined 

 with the suggestion of Hooke, the whole mystery vanished. 

 The application was made by Young himself, and all the 

 principal laws of the phenomena were readily and simply 

 explained. 



(152) Let man be the course of a ray reflected at the first 

 surface of a plate ; mopo'n' that of the 

 ray reflected at the second surface, 

 and twice transmitted through the 

 first. From the point o' let fall the 

 perpendicular o'r' upon the reflected 

 ray on ; it will be also perpendicular 

 to 0V, and will therefore be parallel 

 to the front of the two reflected waves. 

 Now let us conceive a wave reflected at the first surface, in 

 the position o'r', to meet at the same place an anterior wave 

 reflected at the second surface, and let us calculate the origi- 

 nal interval between them. From the time that they reached 

 the first surface at 0, one has travelled over the space or', 

 and the other over the space op + po'. But, if we let fall 

 the perpendicular or upon pb', it is evident from the law of 

 refraction that the spaces or' and o'r are traversed in the 



