REVERSED AND NON-REVERSED SPECTRA. 



149 



commotion will occur in the laboratory or without, tending to make the count 

 uncertain. The rings disappear temporarily during the tremor. In a quiet 

 laboratory, however, and with sunlight replacing the arc light, this would be a 

 method of precision. Thus, for instance, at the highest exhaustions used, over 

 goo fringes would have to pass the D line, a datum from which 7 could be 

 accurately obtained. 



TABLE 18. Values of 7. Iron gas-pipe, i inch internal diameter. =952. 6. 



e= 143.4 cm. i+*=l.O34l. 



If we compare the mean results for 7 with the exhaustion used (pressure p 

 in the vacuum chamber, full barometric pressure po in the air-chamber), the 

 results decrease slightly as the vacuum is higher. Thus 



If p= 58 cm. 

 Then 7= 1.23 



48 cm. 39 cm. 30 cm. 

 1.18 1.15 1.14 



21 cm. 



which is what might have been expected, except that the rate of decrease is 

 much less than would be surmised. There seems thus to be no objection to 

 the use of high exhaustions, which in turn give a better value of 7 from the 

 large range of AJV obtained. 



The low mean value of 7 obtained has been referred to the resistance of 

 the inch piping to the outflow of air. It is probably not due to the stop- 

 cock, as incidental differences in the speed of opening and closing would 

 otherwise have shown a marked effect. One may conclude that the air in 

 the long inch gas-pipe expands adiabatically with a coefficient 7 between i . i 

 and 1.2, in case of such exhaustions as the above. 



