48 



INFRA-RED TRANSMISSION SPECTRA. 



pare the energy that affects the normal eye to the total energy. This would 

 lower the present efficiencies of sources that are rich in the red, which 

 color affects the eye but little. 



The use of iodine in solution is prohibited by the selective absorption 

 of heat rays by all known solvents. On the other hand, solid iodine 

 evaporates readily, so that it would be impossible to keep a solid film of 

 uniform thickness for any great length of time. 



ASPHALTUM. 



(Curve c, f = o.i mm ; = 0.0005 mm.; 6 = 0.03 mm.; d = 0.005 mm.; fig. 31. For the complete 

 curve, to 14 /x, see Carnegie Publication No. 35, p. 75, and fig. 44, p. 198.) 



The next best substance to iodine for absorbing the visible and trans- 

 mitting the infra-red is asphaltum, which is a solid hydrocarbon, previously 

 examined. This substance can be formed into films of any desired thick- 

 ness. In fig. 31 curves a and b are due to Nichols. 1 From the curves it 



100% 



Z 3JU. 4 



Fig. 31. Asphaltum. 



7U 



will be seen that a thickness can be selected which fulfills the conditions 

 of complete transparency beyond 0.7 jx better than any other common 

 substance. Even the thick film (curve c, / = o.i mm.), which transmitted 

 a trace of red, shows great transparency, after passing beyond the effect 

 of the absorption band in the visible. A film absorbing up to 0.65 or 0.7 ;i 

 would be practically transparent in the infra-red (see Carnegie Publica- 

 tion No. 35, p. 17), where, for a very thin film of water, the small absorption 

 band at 4.75 /1, similar in intensity to those of asphaltum, has entirely 

 disappeared. By using a clear rock-salt plate, covered on both sides to 

 prevent the action of moisture, a fair standard could be produced. The 



1 E. L. Nichols: Phys. Rev., 14, 204, 1902. 



