by Reflection from the Pole of a Magnet. 323 



of the coils weighs 14 pounds ; and the wire makes about 400 

 turns. The particular coil employed in any case is put into 

 circuit (generally as a double wire of 200 turns) with a small 

 Grove's battery of only six cells ; and this is the highest power 

 applied in my experiments. In the circuit is placed also a 

 commutator, which is at my hand, so that, while I watch the 

 polariscope, I have the magnetic state of the core under perfect 

 control. 



4. Polar surfaces, — These were originally well planed, and 

 perpendicular to the axes of the cores. For the present pur- 

 pose they had to be smoothed and brightened by polishing, a 

 process which I found troublesome and excessively tedious, 

 from the refractory nature of the material. The polishing 

 was done with fine dry emery powder, applied by cfyamois 

 leather to one of the surfaces, and by a rubber of fine silk stuff 

 to the other. Each rubber was backed by a flat and smooth 

 block of iron, which was worked carefully by hand over the 

 end of the core. The last stage of the polishing was similar 

 to the earlier stages, but without new additions of emery. 

 When the process was finished, each polar surface (though 

 not such a speculum as would satisfy an optician) acted as a 

 pretty good plane metallic mirror, its plane perpendicular to 

 the axis of the core. Placed in a room in ordinary daylight, 

 each mirror gave good regular images of all surrounding 

 objects that were in any degree illuminated ; and in a darkened 

 room, the image of a neighbouring candle-flame was generally 

 very good, brilliant, distinct, and sharply and truly outlined, 

 except towards the rim of the mirror, parts not used in the ob- 

 servations. The surface that had been finished by chamois 

 leather was rather more brilliant than the other, but not so 

 perfectly well planed. 



I should say here that, from all that I have seen in these 

 experiments and in some earlier trials, I consider the finest 

 attainable polish very desirable. In my present apparatus, I 

 would prefer a much finer polish to any increase whatever of 

 magnetic power (3). 



5. Placing of the pieces. — The electromagnet is placed on a 

 solid table, near the edge, and is inclined with its polar surface 

 towards the light by means of a small block placed under the 

 stand. The source of light is a paraffin-flame, narrow and 

 very brilliant, distant a foot or less from the polar surface. 

 Close to the flame stands the first Nicol. The beam of plane- 

 polarized light so rendered is incident horizontally (at an angle 

 of 60° to 80° to the normal) on the polar surface, and is regu- 

 larly reflected. On this side of the polar surface, a few inches 

 distant, comes the second Nicol, which is supported on a lateral 



Y2 



