454 



ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 



tance. One spark is sufficient. The experiment should 



be made in a dark room. 



In Fig. 98, a a, the metal photographic plate ; Z>, <?, the 



brass balls connected with 

 the Leyden - phial. The 

 spark passes between them 

 an( ^ *ke met al plate. At 

 dj e, the effect is shown, and 

 again more plainly in Fig. 

 99. 



On developing it will be 



Fig. 99. 



found that on the point 

 which received the spark 

 there is a blue-white spot about one fortieth of an inch 

 in diameter (Fig. 99). Immediately around this an an- 

 nular space which is perfectly black, 

 the rays of the spark having there had 

 no action ; then follows a white ring, 

 and then another black one. Finally 

 succeeds a whitish stain of an indistinct 

 circular form, which can be traced by 

 inclining the plate as having a diame- 

 ter of about 1| inches. 



That part of the plate from which the spark escaped 

 shows a repetition of the same phenomenon. 



How shall we account for the production of these al- 

 ternate white and black spaces rings of action and in- 

 action ? Some persons might at first be led to suppose 

 that this is only an interesting form of Priestley's exper- 

 iment of u the fairy rings," formed by receiving the shock 

 of a battery on a polished steel surface, when, by the ox- 

 idation that ensues, a film is formed of variable thickness, 

 and giving the colors of thin plates. But a little con- 

 sideration will show that this is impossible, and the facts 

 are only to be explained on the principles of interference. 



