DISPERSION OF " A T " RA YS 59 



I had recourse to Newton's rings. These 

 being produced, in yellow light, for instance, if 

 one passes from one dark ring to the following, 

 the variation of optical retardation in air is one 

 wave-length of yellow light. If, now, with the 

 same apparatus and the same incidence, rings 

 are produced by means of " N " rays, and the 

 number of these rings comprised between two 

 dark rings in yellow light is counted, we shall 

 obtain the number of times which the wave- 

 length of " N " rays is contained in the wave- 

 length of yellow light. This method, applied 

 to rays of index 1-04, gave the values 0*0085 

 instead of 0*008 1 found by the gratings ; and 

 for the index 1-85, the value 0*017 instead of 

 0*0176. Though the ring method is inferior to 

 the grating method, on account of the uncer- 

 tainty attending the exact position of the dark 

 rings in the experiment, an uncertainty which 

 is due to the necessity of rendering these rings 

 very wide, the concordance of the numbers 

 obtained by the two methods constitutes a 

 valuable control. 



In the tables given above I have retained all 

 the decimals occurring in the calculation of the 



