118 



C. Barus — Displacement Interferometer. 



are known. It is surprising to note bow closely these observa- 

 tions lie on a straight line. They do so quite within the errors 

 of observation. 



Fig. 4. 



The slope of the line is 



70*0 — 2 



210 



= -320 



i. e., *32 cm of mercury at 0° C, per ring vanishing ; or, a little 

 over 3 rings (3*125) per cm. of mercury. The mean wave 

 length in question (between the D aud E line) is about 

 X — 55*8 X 10" 6 . Hence, as the effective barometer was 



76-95 cm --50 = 76-45 cm and the tube length 23'8 cm . 



16-45X21-9X10" = 1HK)0280fl 



r -32X23-8 



The datum obtained from the displacement of ellipses at 

 the same temperature (unfortunately not taken) was 1*000279. 



