XVI. On the Composition and Resolution of Streams of Polarized Light from 

 different Sources. By G. G. Stokes, M.A., Fellow of Pembroke College, and 

 Lucasian Professor of Mathematics in the University of Cambridge. 



[Read Feb. 16 and March 15, 1852.] 



When a stream of polarized light is decomposed into two streams which, after having been 

 modified in a slightly different manner, are reunited, the mixture is found to have acquired 

 properties which are quite distinct from those of the original stream, and give rise to a num- 

 ber of curious and apparently complicated phenomena. These phenomena have now, however, 

 through the labours of Young and Fresnel, been completely reduced to law, and embraced in 

 a theory, the wonderful simplicity of which is such as to bear with it the stamp of truth. But 

 when two polarized streams from different sources mix together, the mixture possesses pro- 

 perties intermediate between those of the original streams, and none of the curious phenomena 

 depending upon the interference of polarized light are manifested. The properties of such 

 mixtures form but an uninviting subject of investigation ; and accordingly, though to a certain 

 extent they are obvious, and must have forced themselves upon the attention of all who have 

 paid any special attention to the physical theory of light, they do not seem hitherto to have 

 been studied in detail. 



Were the only object of such a study to enable us to calculate with greater facility the 

 results obtained by means of certain complicated combinations, the subject might deservedly 

 be deemed of small importance. For the object of the philosopher is not to complicate, but 

 to simplify and analyze, so as to reduce phenomena to laws, which in their turn may be made 

 the stepping-stones for ascending to a general theory which shall embrace them all ; and when 

 such a theory has been arrived at, and thoroughly verified, the task of deducing from it the 

 results which ought to be observed under a combination of circumstances which has nothing 

 to recommend it for consideration but its complexity, may well be abandoned for new and 

 more fertile fields of research. But in the present case certain difficulties seem to have arisen 

 respecting the connexion between common and elliptically polarized light which it needed 

 only a more detailed study of the laws of combination of polarized light to overcome; and 

 accordingly the subject may be deemed not wholly devoid of importance. 



The early part of the following paper is devoted to a demonstration of various properties 

 of elliptically polarized light, and of oppositely polarized streams. When two streams of 

 light are called oppositely polarized, it is meant that, so far as relates to its state of polari- 

 zation, one stream is what the other becomes when it is turned in azimuth through 90°, and 

 has its nature reversed as regards right-handed and left-handed. Most, if not all, of these 

 properties have doubtless already occurred to persons studying the subject, but I am not aware 

 of any formal demonstrations of them which have been published ; and indeed some artifices 

 were required in order to avoid being encumbered in the demonstrations with long analytical 



