Non-reversed Spectrum Interferometry. 411 



Thus for the second set (mean) 



r=3'6° 4-8° 5-9° 6'8° 



10 6 A = 6-7 6-4 6-9 6-5 cm 



The wave-length thus comes out very much too large, but in 

 consideration of the inadequacy of the fiducial position, » = 0°, 

 this is not unexpected. Thus the probable values of i (com- 

 puted from X correct) agree with some of the individual series. 

 In addition to this the effect of slightly wedge-shaped plates, 

 etc., can not be ignored. For the first set (mean values), the 

 results are similar. 



C. An air compensator was now installed consisting of a 

 tube e — 15 cm long and about 2 cra in diameter, closed with glass 

 plates. The fringes were easily found, and sharp. Unfortu- 

 nately the pump was not quite tight, so that on breaking the 

 count of fringes at low pressures, it was difficult to state when 

 the conditions had become isothermal. Hence the following 

 results are rough : 



Temp. 19-7. 



No. of fringes, 30 67 30 70 40 68 



Exhausted to (p), 75-1 41-8 75-1 43-0 75'1 32-1 cm 



dp/dn, ... li-i n-i ... io-7 10-6 ... 10-7 11-2 



X X 10 s , ... 59-7 60-0 ... 57-6 57'1 ... 57'8 60-0 cm 



the mean value thus appears as \ = 10~ 6 X 5*88 for sodium 

 light. The equations used are (1) n\ = e(ix — 1) where n is 

 the number of fringes counted, e the tube length and /i the 

 index of refraction of air. Again 



p = C(,,-l)B (2) 



where p is the pressure, & the absolute temperature and the con- 

 stant C computed from normal conditions (76 cm and 0° G) is 

 (Mascart's values) C = 952-6. Hence 



e p e dp e dp 



(3) 



G tiB G d(n$) C3 dn 



when # is constant. It is this assumption which is not quite 

 guaranteed above. To obviate this in the following experi- 

 ments, the total number of fringes were counted from exhaus- 

 tion to plenum. Their number was definite to the fraction of 



a fringe. 



Temp. 19-3° C; e = 15 



No. of fringes, 6 9 5 



Exhausted to (p), 75-8 



dp/dn, 1-090 



A X 10", ... 58-8 







69'5 







69-5 



75-8 







75-8 



cm 







1-090 





1-090 





58-8 





58-8 



