MEMOIR XXX.] ON BURNING GLASSES AND MIRRORS. 453 



which undoubtedly arise from the interference of the in- 

 cident light with the light reflected from the metal at 

 the back of the film. They are the colors of thin plates, 

 like those of a soap-bubble. 



Now, there is a great difference in the action of light 

 upon these differently colored films, though chemically 

 they are the self-same silver iodide. Some have been un- 

 acted upon ; in them the effect of the incident light has 

 been destroyed or reversed by the effect of the light re- 

 flected from the back of the film. Some have been pow- 

 erfully acted upon ; in them the chemical effect is at a 

 maximum the incident and reflected rays have con- 

 spired. 



If any proof were required that these maxima and 

 minima of chemical effect arise from the superposition of 

 similar or contrary motions, it is found in the relative 

 thickness of the films which have been acted or unacted 

 upon. Those in which there has been maximum action 

 have thicknesses as 2:1; that showing the minimum 

 action is 1J. 



If the daylight and simple spectrum rays be permitted 

 to act together on a daguerreotype plate, the rays of 

 which the times of vibration are as 1,2, etc., aid the day- 

 light; but those of wiiich the times of vibration are as 

 i, 1|, 2|, etc., interfere with it and destroy its effect. 



In these numbers we may discern the suggestion of 

 some very important facts. 



One of the most striking instances of this positive and 

 negative action I discovered in the case of the electric 

 spark. Let there be placed over a daguerreotype plate 

 (Fig. 98) two metal balls, connected respectively with 

 the inside and outside of a Leyden-jar in such a way that 

 the discharge may pass from one of the balls at about 

 half an inch distance to the sensitive plate, and from the 

 sensitive plate to the other ball at about the same dis- 



