342 On Rotation of the Plane of Polarization. 



ments as in the last three experiments, and working under 

 the most favourable conditions attainable, I have often left 

 the two Nicols in position at pure extinction, and tried the 

 effects of the simple operations N and S. I have certainly 

 got distinct effects many times in such circumstances, and 

 assured myself that they were due to magnetizations of the 

 iron mirror by getting them to appear and disappear at the 

 instants of make and break of the circuit ; but the effects 

 were so excessively faint that I could not once characterize 

 them as due to rotation of the plane of polarization. I have 

 no doubt whatever that, with a stronger magnet and a finer 

 mirror, and a more intense light, this experiment would be as 

 satisfactory as any of the preceding. 



32. Twelfth experiment : influence of the Submagnet. — The 

 old wedge C of art. 7 has a slit sawn into it at right angles 

 to the edge, as if to divide the block into two equal wedges. 

 The slit is about -^ inch wide, and terminates at the dotted 

 line drawn across the block C in the diagram of 7. Returning 

 to the diagram of art. 27, the bored block is removed, and 

 the slit block put in its place, its largest plane face on the 

 polar surface, and the slit perpendicular to the plane L C F. 

 Block and core are separated successively by six sheets of in- 

 creasing thickness, tissue-paper, thin writing-paper, drawing- 

 paper, pasteboard, thick card-board, and a quarter-inch plank, 

 each of the sheets being perforated properly at F,& so as to 

 expose the polar surface through the slit. All the other 

 arrangements and the procedure are as in the eighth and 

 ninth experiments. The old effects are obtained under these 

 new conditions, but more faintly at the best. They are 

 certainly strongest with the sheets of pasteboard and card- 

 board, -g 1 ^ inch to ^ inch thick. With the quarter-inch plank 

 they are barely if at all perceptible. With the first and second 

 sheets, the tissue-paper and thin writing-paper, I could catch 

 no trace of the effects. 



Summary of Experimental Results. 



33. When plane-polarized light is reflected perpendicu- 

 larly from the polar surface of an iron electromagnet, the 

 plane of polarization is turned through a small angle in a 

 direction contrary to the nominal direction of the magnetizing 

 current. 



When the light is reflected obliquely, the effect in the 

 polariscope is mixed, partly due to magnetic force, and partly 

 due to metallic reflection ; but in this case, as evidently as in 

 the case of normal incidence, the action of the magnetic force 

 is purely or chiefly photogyric, and the plane of polarization 



