214 Messrs. E. Edser and C. P. Butler on a Simple 



To find the wave-length of the line whose scale-number 

 = 371-2: 



*« = 371-2-90'2 = 281, 



X = 



wXo 910-8 x 5328-5 



n-hr 910-8-1-281 



= 4072-2. 



The true value of this wave-length is 4071*8, giving an error 

 of + *4 tenth-metres. 



The following Table shows the calculated and true values 

 for a number of lines in the above spectrum. It is given in 

 order to indicate the degree of accuracy attainable. It is 

 worth notice that these results were all obtained without the 

 use of a travelling microscope, or in fact any auxiliary ap- 

 pliance other than an ordinary pocket-lens. With the latter 

 it is easy to estimate the position of a line relatively to the 

 interference-scale to within one-tenth of a band. Further, 

 the interference-scale in the present instance was purposely 

 made rather coarse so as to admit of reproduction. With a 

 finer scale a greater degree of accuracy might be attained. 



Scale No. 



Wave-length (calculated). 



True Wave-length. 



Error. 



371-2 



4072-2 



4071-8 



tenth-metres. 

 + •4 



286-5 



4383-7 



4383-6 



+ •1 



281-1 



4405-2 



4404-8 



+ •4 



278-6 



4415-2 



4415-3 



--1 



354 



4131-8 



4132-2 



+'4 



4131 



3933-9 



3933-5 (K) 



4-4 



When it is required to determine the wave-length corre- 

 sponding to a great number of spectral lines a graphical 



method may be employed. If we write — =L = the fre- 



quency of the light vibrations, we obtain the simple relation 



n ■+■ v 



— I — = constant, or L = K(?' + n), i.e. the relation between 



r and L may be expressed by a straight line. 



Plotting the frequencies vertically, and r horizontally, we 

 obtain fig. 3. It is only necessary to mark off the scale- 

 divisions, starting from zero, along the horizontal axis, and 

 to mark off vertically above their respective scale-divisions 

 the frequencies of the two standard lines, joining the extre- 



