2 John Mills 



the disappearance of the interference phenomena previously ob- 

 servable. The addition of two similar pieces producing a dif- 

 ference of 180 caused their reappearance. 



The discovery of the electromagnetic rotation of plane polarized 

 light by Faraday was followed by many investigations in that 

 line. The question as to whether an electromagnetic field in- 

 fluenced natural light in a manner analogous to its rotation of 

 plane polarized light was first set forth by Sohncke in the paper 

 previously referred to. In a paper on the nature of the Faraday 

 effect D. B. Brace 1 had already remarked that in the case of 

 elliptically polarized light — that is more nearly approaching the 

 form of vibration of natural light than either plane or circularly 

 polarized — the interference bands became .less distinct upon the 

 formation of an electromagnetic field in the medium. By an in- 

 terference system similar tp that of the fundamental interference 

 experiment of Young, Sohncke found that natural light was 

 capable of rotation in an electromagnetic field. 



His method was as follows : Light from a single slit was 

 allowed to fall upon a screen with two small slits close together. 

 Upon the further side of this screen and at a suitable distance 

 interference bands were observable. By placing behind this dou- 

 ble slit a Soleil double quartz plate of the proper thickness and 

 allowing the light from one slit to pass through one side of the 

 plate, while the light from the other slit passed through the other 

 side, there was brought about a rotation of the components of the 

 light used through an angle of 90 and the consequent disap- 

 pearance of the interference bands. In the path of one of these 

 interfering rays and between the screen and the observer war. 

 placed a parallelopiped of Faraday glass surrounded by a coil. 

 In the path of the other ray was placed a similar plate and coil, 

 but drawn back along the line of the beam far enough to allow of 

 the formation of independent electromagnetic fields. For the 

 strength of current used a rotation of 9.2 ° was possible in each 

 coil. The coils were so connected as to give twice the rotation 

 possible for one coil. If, then, natural light is capable of an 



1 lVied. Ann. 26, p. 576, 1885. 



I46 



