Focal Isolation of Long Heat- Waves. 253 



The results of two such series of observations selected from 

 eight series, all of which were in good agreement, are repro- 

 duced in fio-s. 2 and 3. Curve 2 was obtained with the 



Fig. 2. 



12 



10 











" \ 









\ 



A 



V / 



-x 



" \ 





\J 





; : 









50 



100 150 



Fio-. 3. 



— V A —-——»- ' ' . — 



j_ : I _- ' 



50 



100 



150 



thin quartz plates (2 mm.), and curve 3 with the thick ones 

 (7'3 mm.). The ordinates are the micro-radiometer deflexions 

 in millimetres, and the abscissa? are the thicknesses of the 

 air-film, in sodium wave-lengths. 



