REVERSED AND NON-REVERSED SPECTRA. 



137 



at 100 were repeated. The optical measurements were satisfactory, or at 

 least just short of the counting of interference rings for measurement. The 

 arc lamp, moreover, which is unsteady, would scarcely suffice for this purpose. 

 The results obtained were as follows (table 15) : 



TABLE 15. Refraction of air at different temperatures. 



If the mean values of AN and AN' be taken and a computed 



the result is 



01 = 0.00361 



As these experiments were the smoothest and were made under the most 

 satisfactory conditions, they are probably the most trustworthy. I have not, 

 therefore, been able to obtain evidence for a value of a. (between o and 100) 

 greater than the coefficient of expansion of gases, though it must be confessed 

 that the method in its present surroundings is not sufficiently sensitive to 

 furnish a definite criterion. 



Later results at low temperatures (series 3) like the above series 4, table 

 14, again gave a high result for AN, in each case consistently. It is probable 

 that the interference pattern changes between the case of a plenum and of 

 highly exhausted air, owing either to flexure of the glass ends or to some other 

 cause, or possibly depending only on the form of the pattern which happens 

 to appear. In such a case the lines of symmetry for N (plenum) and N (ex- 

 haustion) would differ, introducing a systematic error very difficult to obviate. 

 Thus different values of AN often follow a difference of adjustment of the 

 mirror at the micrometer, while all cases for the same adjustment are practi- 

 cally identical. 



82. Experiments at red heat. To investigate the feasibility of such experi- 

 ments, an inch steel tube (bicycle tube), 68 cm. long, with flanges brazed on 

 at the ends, and an exhaustion tube near the middle, was heated in an organic 

 combustion furnace to low red heat. The ends just projected outside the 

 furnace and were closed by plate-glass windows with a jacket of asbestos 

 between (applied wet and dried); or, finally, with a jacket of aluminum 

 cement, clay, plaster, etc. These short but relatively cold ends are, of course, 



