$8 "N n RAYS 



plane-mirror being fixed to the arm. Moreover, 

 one measures, not the distance between two 

 consecutive fringes, but that between two sym- 

 metrical fringes of a high order for example, 

 that between the tenth fringe on the right, 

 and the tenth fringe on the left. From these 

 measures of angle, and from the number 

 of lines per millimetre of the grating, the 

 wave-length can be deduced by the known 

 formula. 



Each wave-length has been determined by 

 three series of measures, effected with three 

 gratings, having respectively 200, 100, and 50 

 lines per millimetre. 



The following table exhibits the results of 

 these measures : 



Wave-lengths. 



Being desirous of controlling these deter- 

 minations by the use of a quite different method, 



