322 Dr. J. Kerr's Experiments on the 



oppositely polarized. His immediate object in this fine ex- 

 periment, and in some other work which he has mentioned but 

 left undescribed, was to dispose of doubts that had been ex- 

 pressed by others, as to the reality of the double refraction 

 to which the new phenomena were referred by Brewster*. 



The next name that occurs prominently in the history of 

 the subject is that of M. Neumann, who constructed a mathe- 

 matical theory of this action, based partly on Fresnel's general 

 theory of double refraction f. For a critical view of this 

 theory, and of the experiments adduced in connexion with it, 

 I may refer to Verdet's Optique Physique % ; but, speaking for 

 myself, and from an experimental standpoint, 1 must observe 

 that Neumann's conclusions are certainly contrary to fact. 



M. Wertheim, resuming Brewster's enquiry with improved 

 means and arrangements, experimented on the subject elabo- 

 rately, and with great care§. He secured a sensibly homo- 

 geneous strain, and a uniform optical field, by a proper 

 distribution of compressing and stretching weights ; and he 

 added much to the delicacy of observation and measurement, 

 by the employment of the sensitive tint, and of monochromatic 

 light. He experimented also, as Brewster had already done, 

 upon other media besides glass. Among the most important 

 of Wertheim's general conclusions are the following : — 



(1) The optical effects of dilatation and compression are 

 contrary ; and they are equal when the strains are equal. 



(2) Intensity of strain and quantity of optical effect are 

 sensibly proportional to each other. 



(3) The absolute value of the difference of retardations of 

 the two acting rays is independent of the wave-length ; and 

 the differences of phase are therefore inversely as the wave- 

 lengths. 



There are several important and interesting pieces of work, 

 related more or less closely to what precedes, which have 

 been done by Seebeck, Biot, Kundt, and other able experi- 

 menters; but any large extension of this historical sketch 

 would be out of place. 



I have lately spent a good deal of time in this difficult and 

 interesting field ; and 1 proceed to offer a few notes of my 

 experiments. For a condensed view of the questions pro- 

 posed and results obtained, I need only refer to the enuncia- 

 tions of the successive propositions throughout the paper. 



2. Jamin's Interference-Refractor for Polarized Light. — 

 This is the principal measuring-instrument used in the experi- 



* Oeuvres cFAugustin Fresnel, tome i. p. 713. 



t Poggendorff's Annalen, liv. p. 449 (an. 1841). % Tome ii. p. 373. 



§ Annates de Chimie et de Physique [3] xii. p. 96, &c. (an. 1854). 



