40 REPORT — 1876. 



Siir Ie$ Usagei da Revolver Photo jrapJdque en Astronomie et en Biologic. 



By Dr. J. Janssen, 



Photographies du Pasmge de Venus a Kohe. By Dr. J. Jansse\. 



Sur le Mirage en Mer. By Dr. J. Janssen. 



On Solar Photography, with reference to the History of the Solar Surface. 



By Dr. J. Jajtssbn. 



On the Eclipse of the Sun observed at Siam in Ajiril 1875. 

 Bii Dr. J. Janssen. 



On Rotation of the Plane of Polarization by Reflection from a Magnetic Pole. 

 By John Kerr, LL.D., Mathematiccd Lecturer of the Free Church Training 

 College, Glasgow*. 



In these experiments a beam of light is polarized by a first Nicol, reflected regu- 

 larly from the end of an electromagnetic core of soft iron, and analyzed by a second 

 Nicol. The magnetic force is concentrated intensely upon the miiTor by means of 

 a massive wedge of soft iron, which is separated from it by a narrow chink. The 

 light is incident upon the polar mirror at an angle of C0° to 80°; the plane of 

 polarization coincides with the plane of incidence ; and the two Nicols are exactly 

 crossed, so that the reflected light is extinguished by the second Nicol. 



First Experiment. — When the iron mirror is intensely magnetized as N. pole or 

 S. pole, the light is distinctly restored from pure extinction, to disappear at once 

 when tlie circuit of the magnetizing current is broken. 



Second Experiment. — The first Nicol is turned from its initial position through 

 an extremely small angle— (1) to the right, (2) to the left (from the point of inci- 

 dence on the iron mirror as point of view), so that the reflected light is restored 

 very faintly through the second Nicol. When the mirror becomes an intense S. 

 pole, the eftects of rotations (1) and (2) are strengtheued and weakened respectively; 

 on the contrary, when the miiTor becomes an intense N. pole, the effects of rota- 

 tions (1) and (2) are weakened and strengthened respectively. 



In the two remaining experiments the optical ellects of "the preceding rotations 

 (1) and (2) and of magnetizations S. and N. of the mirror are compensated sepa- 

 rately. The compensator is a slip of plate-glass, held in a standard position between 

 the mirror and the second Nicol and strained by the hands. The angle of incidence 

 is about 75^. 



Third Experiment. — The first Nicol is turned from its initial position through an 

 extremely small angle— (1) to the right, (2} to the left, so that the light is faintly 

 restored from extinction by the second Nicol. The eftects of displacement (1) 

 and (2) are compensated, dowu to pure extinction, by tension and compression 

 respectively. 



Fourth Experiment. — A repetition of the first, with addition of the compensator. 

 The eftects of magnetizations S. and N. of the mirror are compensated, down to 

 pure extinction, by tension and compression respectively. 



The case of perpendicular incidence was tried carefully, but gave no good effect, 

 the arrangements being comparatively imperfect. From" the facts observed, it fol- 

 lows evidently that when a beam of "plane polarized light is reflected from a mag- 

 netic pole, the plane of polarization is turned in the process of reflexion — to the 

 left by a south-seeking pole, to the right by a north-seeking pole ; so that in this 



* A full account of this investigation is given in the Phil. Mag., Mny 1877. 



