REVERSED AND NON-REVERSED SPECTRA. 145 



One may note that the results for 7, when rings are counted, are consistently 

 too large, but always of the same order. In fact, if R were increased by the re- 

 duction factor To76/2 73^0, the values of 7 would all be nearly correct ;but there 

 is no reason for such a correction. Moreover, since the data for 7 obtained 

 from AW (ellipses brought back to fiducial position) and from R (ellipses dis- 

 placed) are each separately consistent with each other, the discrepancy can 

 not be due to leakages of air, as these would affect both measurements in the 

 same way. The only source of error which is not common to both (apart from 

 the displacement of ellipses) is the possible distortion of the glass upon exhaus- 

 tion ; for, in case of A/V, measurement is made at a plenum and at maximum 

 exhaustion only, but at varying pressures for the case of rings. Thus if the 

 rings needed are supposed to increase in the ratio of 



pa/p=i.$ 1.6 2.0 2.5 



roughly, an approximate adjustment of the two sets of observations would 

 also be obtained. Moreover, the effect of flexure would be an increase of the 

 path of the beam in glass and so counteract the negative effect of decreased 

 density. 



88. Effect of strained glass. To detect the possible effect of the inward 

 flexure of the two plates of glass, a metallic ring about 25 cm. in internal 

 diameter was provided. To this, two glass plates of about the same thickness 

 (0.8 cm. each) as in the above vessel were cemented free from leakage and 

 kept in place by clamps. The distance apart of the two plates within was 

 but 1.8 cm., so that the micrometer displacement due to exhaustion of air 

 was reduced to a small value. Hence, if the flexure of the glass plates due to 

 exhaustion and the reverse were optically appreciable, it should here be 

 detected. 



To compute the residual air effect for the lamella of air, e= 1.8 cm. thick, 

 we may write 



(1) C# O =/(M-I)=A/(JUO-I) 



where (7 = 952.6, t?o is the temperature of the isothermal experiment, n and 

 /z the index of refraction of air at the pressures p and p . Furthermore, 



(2) M I=MO i A-/V/2 



if A1V is the micrometer displacement for the pressure difference p p at # . 

 Finally, if n is the number of rings vanishing or of fringes passing at the 

 sodium line, then 



(3) AAT = n^ 



2Xl0 6 



Thus if p-p = dp, then 



6 



= o. 216 



2Xio 6 e 



58.93 



