40^ FOURTH REPORT — 1834. 



of the polarizing plate in these phenomena. To account me- 

 chanically for the fact of the non-interference of the two pencils, 

 when the liglit incident upon the crystal is unpolarized, it is 

 necessary to consider such light as a rapid succession of systems 

 of waves polarized in all azimuths ; so that if any two planes be 

 assumed at right angles, there will be an equal quantity of light 

 actually polarized in each. Each of these portions, when re- 

 solved into two within the crystal, and these afterwards reduced 

 to the same plane of polarization by the analysing plate, will ex- 

 hibit the phenomena of interference. But the interval of retard- 

 ation differs by half a wave in the two cases ; the tints produced 

 therefore will be complementary, and the light resulting from 

 their union will be of a uniform whiteness. 



We are obliged to admit, therefore, that common light con- 

 sists of a rapid succession of systems of waves, in each of which 

 the vibrations are different. But the phenomena of interference, 

 (which are exhibited by common light) compel us also to admit, 

 as Professor Airy has observed*, that the vibrations do not 

 change continually, and that in each system of waves there are 

 proba1)ly several hundred successive vibrations, which are all 

 similar, — although the vibrations of one system bear no relation 

 to those of another, and the different systems succeed one 

 another with such rapidity as to obliterate all trace of polariza- 

 tion. This persaltum transition from one system of waves, to 

 another in which the vibrations are wholly different, seems a 

 complication in the machinery of light, for which the elegant 

 simplicity of the parts better known has not prepared us ; and 

 I cannot but indulge the hope that the hypothesis, Avhich now 

 stands as the representative of experimental laws, may be found 

 to merge in some simpler physical principle. 



The laws of interference of polarized light have thus supplied 

 the defective link in the explanation of the colours of crystallized 

 plates first suggested by Young. The magnitudes of the re- 

 solved vibrations are known, when the planes of polarization of 

 the two pencils are given with respect to the plane of primitive 

 polarization, and the plane of analysation ; and as the laws of 

 double refraction enable us to find the interval of retardation of 

 these pencils, we have all the data necessary for the compu- 

 tation of the intensity and colour of the light resulting from 

 their interference. This computation has been given by Fresnel, 

 not only for a single plate, but likewise for two plates super- 

 posed f J and the theory has been since more fully developed by 

 Professor Airy J. The results are found to be, in all cases, in 



• Mathematical Tracts, p. 407. f Annales de Chimie, torn. xvii. 



1 Cambrhlge Transacttous 1831, and Math. Tracts. 



