768 Prof. H. Rubens and Mr. H. Hollnagel on 



we are dealing with two radiations of different wave- 

 lengths ; it is also of significance to notice that the two 

 bands are of various intensities, as is evidenced by the non- 

 extinction of the maxima and minima. The wave-length Xj 

 of the stronger ray is obtained most precisely by dividing 

 the distance A, between " homologous " maxima or minima 

 by ft, the number of half waves in that interval and multi- 

 plying by the fourfold value of the drum-division in /j,. 

 Under "homologous" maxima and minima are to be under- 

 stood such points as have a corresponding phase. In fig. 4 

 the maximum a corresponds with the minimum i' and the 

 maximum s, just as does b with Jc! and t ; further examples 

 are a 1 with k and s', V with I and t' &c. As it is not in 

 general probable that the relation between the wave-lengths 

 of the two rays shall be an integer of small magnitude (in 

 present case 15 : 17), we are in a sense dealing with an 

 approximation. However, an error in the coordination of 

 the corresponding points causes but a slight change in the 

 value of the wave-length of the more intense stripe. 

 Whether the radiation of lesser intensity is of greater or 

 lesser wave-length than that of the stronger band can be 

 determined as follows : — In that portion of the interference 

 curve where both rays are approximately in phase, a 

 half wave corresponds to a wave-length of mean value 

 which is greater than would be obtained from the shorter 

 wave were it present alone, and less than the value 

 which would be given by an interference curve produced 

 by the longer wave alone. If this mean value of the 

 half wave I is divided into A, the distance between cor- 



A 



responding points, a number, B = y, results which is greater 



than n, if the weaker stripe is present as a shorter wave, and 

 less than n if present as a longer wave radiation. In either 

 case the wave-length is 



ft -" 



a,2 = A/;l or A/2 =: A<i 



n + -2 ' 'n-2 



= -r9 20 ' 91 > t or =^9 20-91/* 



ft+2 n— 1 



respectively. From this it is evident that the wave-length 

 of the weaker stripe, X 2 , is influenced to a much greater 

 extent than is X x by the choice of the corresponding points, 

 i. e. by the value of n. 



A view of the curves also indicates a method of recog- 

 nizing which of the two instances above mentioned is present, 



