REVERSED AND NON-REVERSED SPECTRA. 



139 



ends of the tube and the leakage at the windows both contribute to a low 

 value of t' ', But these do not seem to be adequate reasons. It is more probable 

 that the longitudinal radiation of the air on the one hand and the value of 

 i /a = 2 73 assumed (if this is too small) may be the chief causes for the low 

 value of t'. It is not, of course, possible to come to any further decision; but 

 the experiments are distinctly unfavorable to the large value of a (small 

 i /a) above considered. 



The method is not adapted for very high temperatures, since equation (7) 

 may be written 



and therefore, since r'AAf' = 



where (T referring to absolute temperature) AN' rapidly reaches the limit of 

 accurate measurement. 



83. Further experiments at high temperatures. A variety of experiments 

 were now made to obtain a more nearly tight joint at the ends, by using 

 various clays, aluminum, etc., as cements, but 

 without success. Finally, an improvement was 

 obtained by using plaster of paris in the way 

 shown in figure 94. A is the end of the hot 

 tube in the combustion furnace F. The flange / 

 is set somewhat back, so that packing of plaster 

 p may secure the window g to the end of the 

 The plaster is put on wet and allowed to 



94 



tube. 



dry thoroughly. Lying outside of the furnace, 



it is never heated to redness. The joint is at 



first fairly good, though it gradually deteriorates at high temperatures, and 



must be replaced. In this way the following results were found: 



Just below red heat 



p. 

 74.5 



12.4 

 12.5 



12.0 



TABLE 16. 



Cold tube (22) 



p. I0 3 XAAT 

 74.0 17.6 Low red heat 



17-7 

 17.8 

 18.1 

 17.8 



p. 



73.5 



8.9 

 8.5 

 8.6 

 8.8 



Thus, from the first and second series, t 1 '= 154; from the first and third series, 

 ' = 330. As in the first experiments tried, both of these data are much too 

 low. Here they can hardly be referred to the leak, since this was smaller. 

 The ends are exposed not more than i or 2 cm. each, or a total length of about 

 70 cm. of tube. 



Some adjustment is needed at the mirrors, to place the slit images in coinci- 

 dence for the case of an exhaustion, as compared with a plenum of air. This 

 adjustment is slight, but unfortunately its effect on AN' can not be estimated. 



