REVERSED AND NON-REVERSED SPECTRA. 



153 



in the preceding table. Hence this discrepancy has been eliminated. What 

 is most interesting is that the 4-inch tube shows no consistent difference in 

 the 7 values for high or low exhaustion. Thus the mean values under increas- 

 ing exhaustion, p, are 



= 56.8 

 7= 1-35 



47.6 

 1.36 



38.7 

 i-3S 



29.7 

 1.36 



20.7 

 1.38 



Accidentally the highest value of 7 belongs to the highest exhaustion. 



The chief anticipation of the work (i.e., that with a 4-inch tube the true 

 value of 7 would appear) has not been fulfilled. The value obtained is still 

 much below normal, successive results ranging as follows: 



Diameter of tube 2.5 5.0 10.0 cm. 



Mean 7 1.17 1.29 1.36 cm. 



Diameter of exhaust pipe 2.5 5.0 5.0011. 



The relatively small increase between the tubes 5 cm. and 10 cm. in diam- 

 eter is disappointing. At the rate obtained from the first two experiments 

 (see fig. 95) a 3-inch tube should have been nearly sufficient. At the rate 

 established by the last two observations, however, a tube at least 5.5 inches 



1-0 







95 



in diameter would be needed to obtain trustworthy values of 7. These differ- 

 ences are possibly due to the exhaust pipe, which in case of the last observation 

 does not increase in size. Hence a 3-inch pipe with a 4-inch stopcock may 

 be estimated as being adequate for 7 measurement, provided the exhaust pipe 

 is straight and clear throughout. 



The observations were broken off at this point, with the object of searching 

 for some means of obtaining a more sensitive and regular interference pattern. 

 If the method is to be ultimately successful, then icr 4 cm. on the micrometer 

 must be guaranteed. If the ellipses are not quite regular or not closed, this 

 is not the case. A more sensitive method of defining optic density is thus in 

 question. 



