COLOURS OF THIN PLATES. 71 



But the facts observed by M. Arago and Professor Airy seem 

 to overturn altogether this part of the theory of emission. The 

 rings formed by a plate of air, inclosed between a lens of glass 

 and a metallic reflector, vanish altogether when the light is 

 polarized perpendicularly to the plane of incidence, and is incident 

 at the polarizing angle of glass. Under these circumstances, no 

 light is reflected from the upper surface of the plate ; but as it is 

 abundantly reflected from the lower, the disappearance of the rings 

 proves that the light reflected from the upper surface is essential to 

 their production. That the light reflected from the lower surface 

 also concurs in their formation, appears from the effects observed 

 by M. Arago, when the metallic plate was tarnished ; and we are 

 thus driven to the conclusion, that the phenomena arise from the 

 union and mutual influence of the pencils reflected from the two 

 surfaces. 



This mode of explaining the colours of thin plates was pointed 

 out by Hooke, in a remarkable passage in his Micrographia, some 

 years before the subject was taken up by Newton. In this passage 

 he very clearly describes the manner in which the rings of succes- 

 sive orders depend on the interval of retardation of the second 

 " pulse," or wave, on the first, and therefore on the thickness 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 mathe- 

 matician, was signally unsuccessful. Euler thought, in fact, that 

 the colours of thin plates, as well as those of natural bodies, 

 arose from emitted, and not from reflected light. The incident 

 light was supposed to excite the vibrations of the plate, the fre- 

 quency of which depended on its thickness, in the same manner as 

 the frequency of the vibrations 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 



* Mem. Acad. Berlin, 1752. 



