by Reflection from the Pole of a Magnet. 325 



effect of this arrangement is, that when the circuit of the mag- 

 netizing current is closed, there is a very powerful concentra- 

 tion of magnetic force upon the mirror, and particularly on 

 that part of it which is utilized optically in the experiments, 

 on so much of it, namely, as the chink between wedge and 

 core leaves exposed, on one side to the lamp, and on the other 

 side to the observer's eye. The lines of magnetic force are 

 sensibly perpendicular to the reflecting surface. The iron 

 mirror is a true polar surface; and its intensely contrasted 

 states, as north, south, neutral, are perfectly under control 

 through the commutator. 



The wedge intercepts a large part of the image of the flame. 

 The pieces are generally so placed that the part left of the 

 image is a strong middle segment, both top and bottom being 

 cut off. The object now watched in the polariscope is a broad 

 streak of light, crossing the chink at right angles from top to 

 bottom, very sharply defined, and perfectly suitable as an ob- 

 ject in delicate polariscopic work. 



Splinters of different thicknesses are employed in different 

 experiments, and in variations of one experiment. My only 

 rule is, that the chink between block and core be as narrow as 

 the requirements of the optical observation will allow. On an 

 average, the width of the chink in the following experiments is 

 about 2*o °f an inch. The arrangements now described (3 . . 7) 

 are shown simply in the adjacent diagram. 



S- 



L is the source of light, E the observer's eye, A and B the 

 first and second Mcols, C the wedge of soft iron. 



8. First experiment, — The pieces arranged as in the diagram, 

 the chink between block and mirror as narrow as possible, the 

 plane of polarization of the light incident on the polar mirror 

 parallel or perpendicular to the plane of incidence, and the second 

 Mcol turned into the position of pure extinction. The observer 

 now watches the chink through the second Nicol, and works 

 the commutator. When the circuit is closed, the streak of light 

 immediately reappears. The effect is very faint at the best ; 

 but it is very distinct and perfectly regular, unless the appa- 

 ratus is in some way out of order, the mirror dimmed, or the 

 battery working below its average power. Under ordinarily 



