Focal Isolation of Long Heat-Waves. 



259 



this spectral range, though their transparency is greater 

 here than in many other regions. Water turns out to be 

 more transparent for these rays than for the 82 \ju residual 

 rays from KBr, while the opposite is true for water vapour. 



Table IV. (fluids). 



Material. 



Thickness D. 



Percentage 

 transmission D. 



Benzol 



mm. 



100 



0-loS 



0-158 



0-158 



0-029 



0-044 



per cent. 



50-8 

 7-9 

 371 

 4Crl 

 25-8 

 13-(> 



Ethyl alcohol 



Ethvl ether 



Castor oil 



Water 



Water . 





Table V. 



Material. 



Thickness 



of vapour 



layer d. 



Corresponding 

 thickness of 

 fluid layer d. 



Percentage 

 transmission I). 



Alcohol vapour 



mm. 

 200 

 200 

 200 

 400 

 200 



mm. 

 0-023 

 0350 

 0-0627 

 021 



per cent. 

 •88 

 35-5 



100 

 196 



100 



Ether vapour 





Water vapour 



CO„ 





It is possible, however, that the addition of 10 per cent, of 

 glycerine to the water used for preparing the films used in 

 the previous work with the KBr rays diminished its trans- 

 parency somewhat. 



Both benzol and its vapour are exceedingly transparent, 

 which is in agreement with earlier work upon the optical 

 properties of this substance. 



From Table V. it is apparent that for an equal number of 

 molecules in the path of the rays, the absorption of the fluid 

 is greater than that of the vapour, especially in the case of 

 water. 



For the study of the reflecting power of various sub- 

 stances for these long waves, we modified the apparatus 

 somewhat in order to make perpendicular incidence possible. 



S2 



