144 THE INTERFEROMETRY OF 



place the micrometer with an accuracy of more than 0.0002 cm. or 0.0003 

 cm. in successive cases, A./V being the difference of two readings, each uncer- 

 tain to lo" 4 cm. But the effect of this is to throw out 7 by about the same 

 number of tenths, so that the roughness of values in the table is inevitable. 

 On the other hand, however, 7 obtained by displacement is usually too small, 

 whereas the value computed from the evanescence of rings is always much 

 too large. Thus in the first series there should have been an evanescence of 3 1 

 rings, in the second of about 50 rings, in the third of 64 rings, in the fourth of 

 85 rings. The reason for this discrepancy is very hard to determine, but will 

 be considered in the next paragraph. The mean values of 7 from displace- 

 ment and from rings are usually more nearly correct than either, as if the 

 errors were equal and opposite in the two cases. The error is, in some way 

 which has not been made out, associated with the placing of the micrometer. 

 Thus, without apparent cause, the micrometer reading with a plenum of air 

 may differ by several io~ 4 cm., so that if these discrepancies are in opposite 

 directions the value of 7 shows such large divergences as in series 4, for in- 

 stance. In other words, the error appears to be extraneous to the method of 

 experiment. 



It has been suggested that the number of vanishing rings observed is approx- 

 imately about 10 per cent too small throughout, and that the corresponding 

 data for 7, though excessive, are nevertheless of the same order of value. 

 Experiments were made to determine whether the change of wave-length, X, 

 influenced this result. This was done by allowing the center of ellipses in 

 one case to move from the D line towards the red, in the other from the yellow 

 into the D line. The mean wave-length would in the last case be smaller, and 

 one may estimate the former as 



c-Xp AAT 



where AAfo is the displacement of mirror which passes the center of ellipses 

 from the C to the D line. This was found to be io 3 A7V = 28.1 cm. Hence 



A/V 



X = X D +3-35Xio- 6 X 



O.O2OI 



Even in the final case, therefore, where io 3 A]V=2.5, \ D would not be in error 

 by more than 0.5 per cent. Using sunlight and at 



= 76. 75 cm. = 48. 65 cm. T = 2Q4.5 (abs. temp.) 



the number of rings R were counted when the ellipses traveled into the D 

 line and from the D line, respectively, with results of which the following 

 are examples: 



From D line, R = 47 46 46 Mean ^ = 46.3 



Into D line, R = 4S 47 46.5 46 Mean ^ = 46.1 

 Indifferent, R = 46 46 Mean R = 46.0 



These results agree with the second series of table 17, and there is thus no 

 appreciable difference. 



