from the Equatorial Surface of a Magnet, 171 



the band being fairly preserved through large displacements. 

 In the next two experiments, I make use of this movement of 

 the band as a mere initiation of restoration from pure extinction 

 in the polariscope. I call it a mere initiation of restoration, 

 because the band has already undergone a large displacement 

 over the surface of the mirror, by rotation of the second Mcol 

 or otherwise, before there is any sensible restoration of the re- 

 flected image from pure extinction. 



15. Fifth Experiment. — The plane of polarization of the 

 light incident on the mirror is parallel to the plane of inci- 

 dence; and the extinction in the polariscope is made as pure as 

 possible, the old place of the reflected image being covered by 

 an obscure cloud or band (art. 14). The observations now to 

 be described in this article and in art. 16 have been made re- 

 peatedly at the four incidences 70°, 65°, 60°, 45°, with con- 

 sistent and uniformly distinct results. 



(1) The two Mcols are untouched, and remain in their 

 initial positions of pure extinction. A left-handed current 

 sends the band up through a small distance very clearly ; a 

 right-handed current sends the band down ; contrary currents 

 in rapid succession act accordingly, and give larger displace- 

 ments of the band (art. 7). 



(2) With open circuit, a right-handed rotation of the second 

 Mcol through the position of extinction sends the band down 

 very clearly. Compared with observation (1), this verifies the 

 First Law stated in art. 13, that the right-handed current con- 

 spires with a right-handed rotation of the second Mcol. 



(3) With open circuit, a right-handed rotation of the first 

 Mcol through the position of extinction sends the band down 

 certainly, though the effect is not generally so good as that in 

 (2). The results in (1) and (3) verify the exception to the 

 second Law stated in art. 13, as the present incidences lie be- 

 tween principal and normal. 



16. Sixth Experiment. — The plane of polarization of the 

 light incident on the mirror is perpendicular to the plane of 

 incidence ; other things as in art. 15. 



(1) The effects of magnetization obtained in art. 15 are 

 simply reversed. A right-handed current sends the band up 

 very clearly ; a left-handed current sends the band down; con- 

 trary currents in rapid succession act accordingly. 



(2) With open circuit, a right-handed rotation of the second 

 Mcol sends the band down (exactly as in art. 15). This agrees 

 with the exception to the First Law stated in art. 13. 



(3) With open circuit, a right-handed rotation of the first 

 Mcol sends the band down (exactly as in art. 15). This agrees 

 with the Second Law (art. 13). 



