PHYSICS OF THE NINETEENTH CENTURY. LIGHT. 457 



to c, where of course it would become nil. It will be seen that the 

 distances c o, o L, can be easily measured, and also the angle which 

 A B subtends at c. When this has been done, the accurate calculation 

 of the distances p A and / B will present no difficulty. When homo- 

 geneous light, i.e., light of one colour and refrangibility, is admitted 

 through the slit L, the screen exhibits a number of alternate light and 

 dark stripes as in Fig. 205. The centre c (which is equally distant 

 from A and B) is always light, but at a certain interval right and left 

 of it dark bands, 1,1'; 2,2', etc., are seen. f 



When the distances of the black band i 5 3 ai I* 3 

 from A and B are calculated, they are 

 found to differ by some extremely small 

 distance, which we may call d. The same 

 calculation for the second dark band 2 

 shows a difference in the paths of the rays FIG. 205. 



from A and B of just three times d\ the 



third dark band 3 is placed where the difference of length amounts 

 to 5 d, and so on of the odd multiples of d. Then if the differences 

 for the bright bands be calculated, they are at c = o; at a = 2 d; at 

 b = 4 d ; and so on, following the even multiples of d. What is this 

 small difference d thus continually reappearing in these measurements ? 

 It is perfectly explained by supposing that light is a wave motion, and 

 that the distance d is half the length of a wave. The waves, which 

 enter the slit at L in a certain phase, having to pursue different 

 lengths of paths in order to reach the screen F G, arrive there in dif- 

 ferent phases. Where the lengths of path by which the waves reach 

 the same part of the screen differ by an odd number of half wave- 

 lengths, the undulations arrive in opposite phases, and neutralize or 

 extinguish each other ; hence the dark bands. When this experiment 

 is made with light of the various colours, it is found that the dark and 

 bright bands are closer as the rays are more refrangible, that is, the 

 value of d is less as we pass from red to violet. The following table 

 shows the lengths of the waves corresponding to the various colours, 

 and the number of vibrations or undulations that must take place in 

 a second. 



