"N n RAYS 



the abscissae stand for the wave-lengths and 

 the ordinates for the indices diminished by 

 unity. 



Nature of 

 the rays. 



Each of the divisions marked on the axis 

 of abscissae corresponds to 0*001, and each 

 of the divisions marked on the ordinate axis 

 corresponds to an excess over unity equal 

 to croi. 



In spite of all the care with which the 

 experiments were executed, the deviations are 

 so small, and, consequently, the indices so near 

 to unity, that the table and diagram can only 

 be regarded as a preliminary indication of the 

 behaviour of the dispersion in the very slightly 

 deviated part of the spectrum. An important 

 consequence arises from these measures, viz. 

 points corresponding to " N " rays, and those 

 corresponding to Nj rays, are all situate on the 



