394 OPTICAL PROJECTION 



perature, and more precision as regards points of action, than 

 any thermo-pile I can conceive of. This beautiful measuring 

 instrument might also be employed in the preceding experi- 

 ments, but is so delicate, and so rarely available for general 

 lecture purposes, that I must only refer concerning its con- 

 struction and use to the inventor's own description. 1 



CHAPTER XXIV 



MAGNETISM AND ELECTRICITY 



THE fundamental experiments in magnetism and electricity 

 can be projected in a manner so obvious as not to require any 

 detailed treatment. As a rule the needful preparations will 

 consist simply of reducing the apparatus to a rather small 

 scale, when the projection will take place under the general 

 conditions described in Chapter XIV. Only a few experi- 

 ments need be given here for the sake of illustration, or of 

 various remarks for which they afford occasion, and which 

 may be useful in other instances. 



233. Polarity. A compass needle mounted either upon a 

 small foot, or upon a graduated glass plate, projected with the 

 vertical attachment, will readily show direction, attraction, 

 and repulsion. Two light soft iron wires hung from the 

 same point by silk threads attached to one end of each, in 

 the field of the condensers, as a magnetic pendulum, will 

 also exhibit repulsion. 



With the glass-bottomed trough filled with water in the 

 vertical attachment, and a number of magnetised steel needles 

 stuck perpendicularly in small cork floats, Prof. Mayer's 

 symmetrical figures, formed by such needles when the pole of 



1 Proc. JR. S. xlii. 191, and Phil. Trans, clxxx. 159. 



