FRESNEI/S METHOD. 



205 



Two rays cannot destroy each other unless they have 

 a common origin ; that is to say, unless they both em- 

 anate from the same particle of an incandescent body. 

 The rays from one side of the sun's disk do not interfere 

 with those from the other side, or from the centre. 



Among the thousands of rays of different tints and 

 refrangibilities of which white light is composed, those 

 only are capable of interference which possess colours 

 and refrangibilities identically the same ; thus, in what- 

 ever manner we take them, a red ray will never destroy 

 a green ray. 



With respect to rays of the same origin and the same 

 colour, they are constantly mixed and superposed with- 

 out influencing each other ; they produce effects repre- 

 sented by the sum of their intensities, if at the moment 

 of their crossing each other they have gone through 

 routes perfectly equal in length. 



An interference can alone take place when the routes 

 through which the rays have passed are unequal ; but it 



the focus of a small lens, to be divided into two streams, either by 

 reflection from two mirrors very little inclined to each other, or trans- 



mission through a very obtuse angled prism. Here the interference 

 stripes are seen by the eye-glass in the middle of the mixed light in 

 the greatest purity and intensity of alternation of brightness and 

 darkness. Translator. 



