﻿Rectification of the Spectrum. 949 



therefore be on an axle), and with the independent rotation 

 of the grating actuated by the alidade. In the definite 

 adjustment this limit must coincide both with the spectrum 

 line A, for which the index, ??, is to be found, and with the 

 cross-hairs of the telescope. Three lines, in other words, are 

 to be brought into coincidence; but after a little practice this 

 succeeds fairly well, unless the limit of total reflexion is too 

 faint. 



The difference in angular reading on the two sides of the 

 undeviated ray is the double angle of total reflexion, 2</>. 

 The adjustment in the first spectrum holds equally for the 

 second and third orders ; and it is in the latter that the pheno- 

 menon is liable to be particularly clear and the adjustment 

 correspondingly precise. The beam of white light should 

 illuminate the whole oratino- whose ruled face is verv 

 nearly on the axis of rotation of the alidade. Sunlight is 

 essential for these measurements, since the spectrum lines 

 are to be used. This was not of course necessary in the 

 preceding paragraph. 



The following example of results obtained in this way may 

 be given 



Line. 



20. 



t. 



it. 



n from fig?. 6 &c. 



D.... 



.. 138°-28 



19°0 



1-5210 



J 1-5215 

 \ 1-5206 



E ... 



... 136°-53 



18°- 7 



1-5244 



f 1-5240 



Ll-5250 



which is practical coincidence within the errors of observation, 

 and thus verifies the above work as a whole. 



8. The Dispersion Constants of the Glass of the Grating. — 

 To reduce these data for practical purposes it will be 

 sufficient within the range and relatively to the precision of 

 observations to assume the simplified form of Cauchy's 

 equation, 



n = A + B/A 2 , 

 if he is the index for the E line and \e its wave-length, 



*(i-i> 



from which B may be computed for each pair of values of n 

 and X. B was found separately from each of the curves (like 

 fig. 8). The values of n — n E and X are chosen as near the 

 observations as possible. 



