130 



THE INTERFEROMETRY OF 



These conditions are particularly puzzling, since in 73, with the use of the 

 arc lamp, both results were nearly normal. I therefore endeavored to detect 

 the causes for this difference of behavior. 



TABLE 12. Summary of Table n. "St." refers to standard data. A=(HQ i). 



The standard of length was first compared with a normal meter, showing 

 that the i = 138.0 cm. for the M tube should be replaced by 137.75 cm. and 

 for the N tube by 137.59 cm. As this correction affects all the results, MO i 

 and B, in the same ratio, it contributes nothing to modify the discrepancy 

 in question. The correctness of the micrometer screw was assumed, as it was 

 of good manufacture. 



The test next was made to see if the lines taken as C and F actually had 

 the accepted wave-lengths. A revolvable arm, with its axis at the grating 

 and 125.5 cm - l n g> was therefore installed for the direct measurement of the 

 diffraction of the grating. The results obtained for the wave-lengths of the 

 lines taken showed that no mistake had been made in their selection. 



To endeavor to obtain further evidence, the values of B were computed for 

 the mean data of table n, by using the standard values for MO i in case of 

 the C and F lines and the AN/e given by the observations. The results so 

 obtained are given in the last columns of table 12, for each of the four series 

 and the C and F lines at normal temperature and pressure. The mean results 

 are thus 



#oXio 14 =i.i9 from observations with sunlight directly 



between C and F lines. 



.Z?oXio 14 = 2.25 from standard MO i = .0002918 at C line. 

 5 Xio 14 =i.52 from standard MO i = .0002950 at F line. 



The B values of table 10 show a march to be referred to temperature and 

 pressure. So the present unsatisfactory differences are probably pressure- 

 temperature effects beyond the discrimination of the method. 



One reason for this discrepancy which suggests itself is the possible distor- 

 tion of the glass plates at the end of the exhaust tubes during the exhaustion. 

 There may be a residual temperature effect due to the heating of the air by 

 the beam which passes twice through it, above the indicated temperature of 

 the surrounding tube of iron. But as MO i is already too large compared 

 with standard values, this would make the case worse. Similarly, a larger 

 thermal coefficient than the normal value (1/273) would further increase 



