Non-reversed Spectrtmi Interferometry. . 407 



In this case sin #, = \ / J) t , where D 1 = 352 X 10~ 6 cm. or 

 0\ = 9° 38'; and sin 6\ = \/D„, where D, = 200 X 10" 6 cm. or 

 0'=17° 9', whence 



a= 26° 47' and 8 = 7° 31'. 



In a later series of experiments, the play of the screw, s, 

 was improved, so that it ran more smoothly. The following 

 values were found in two repetitions, from four pairs of 80 

 fringes each : 



he X 10 6 = 30-1, 30-0 cm , 



and in five pairs of 100 fringes each, he X 10° = 29 - 5 0m . 



If the mean value of these data is compounded with the 

 above mean, the average is 



8e X 10" — 29-56 cm . 



5. Continued. Second order spectra. — The same pheno- 

 menon was not sought in the two second order spectra from 

 G'. Magnificent arrows were obtained, useful throughout 

 about 5 millimeters of the micrometer screw, after which they 

 lost clearness. This limited range could no doubt be immensely 

 increased if optical plate glass were employed in place of the 

 ordinary plate used. The data for pairs of observations, includ- 

 ing 60 or 80 fringes, gave (5 repetitions) a mean value of 

 Se X 10 6 = 30 - 5 cm . In the last two measurements the sodium 

 doublets coincided. 



In this case sin t " = 2X / D» where Z> 2 = 200 X 10" 6 cm. 

 and D, = 352 X 10"' cm. (first grating). Thus 



o-— 450 u', 8 — 26° 28'. 



If the above mean data are summarized the results appear 

 as follows (X = 58-93 X 10"' cm.) : 



G 1st order, G' 1st order, mean 8e X 10" = 29'56 cm 



G 1st order, G' 1st and 2d order, mean 8e x 10" = 30-2 



G 1st order, G' 2nd order, mean Se X 10 6 = 30-5. 



If computed as Be = \ / 2 cos 8/2, these become 



Se X 10 6 = 29-53 cm . Diff + -03 cra 



29-Y8 + -42 



30-27 + -23 



The maximum error of 4 X 10~ 7 cm. is equivalent to but a 

 little over 1 per cent of the distance between fringes, and it 

 would be idle to suppose that the apparatus, fig. 4, could be 

 set more accurately. In fact, the largest error occurs in the 

 second set which were first made and in which the play of the 

 apparatus, fig. 4, was inadequately smooth. 



