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SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 68 



(b) Iodine screen : Energy of Xx X transmission / for A^ + scattered 

 energy of A<4/x, greatly diminished by opaqueness of / for A<4/a, 

 + scattered energy of A>4fiX absorption for it of /. 



(c) Iodine and quartz: Scattered energy of A<4/x, greatly di- 

 minished by opaqueness of / for A<4/a, and diminished by reflection 

 from surfaces of quartz. 



(d) Quartz : Scattered energy of A<4 [x, diminished by reflections 

 from surfaces of quartz. 



The point especially to be noted is that the " field-light " is not the 

 same in the two deflections (a) and (b) whose quotient would give 

 the transmission of the iodine screen if there were no field light. 

 Before taking the quotient (b)/(a) it was necessary to subtract 



Fig. 12. — Transparency of iodine film about 0.006 cm. thick. Abscissae are 

 proportional to deviation distances on plate from 1.8 M. The wave-lengths in 

 microns (fj-) are indicated at the top of the figure. Ordinates are percentage 

 transmissions. 



from (b), (c)-f 1.17 (c) -flight scattered from other wave-lengths 

 greater than 4 fi and from (a), (d) -hi.17 (d) -Flight scattered from 

 other wave-lengths greater than 4^ (see section on field light). 

 Due allowance was computed for all field light as discussed in the 

 Appendix II. 



Figure 12 shows graphically the results of the two holographs. 

 Unfortunately the intense heat from the 44-glower Nernst lamp upon 

 the iodine screen only about 10 cm. away, despite the protection of 

 the rock-salt plates caused an evaporation of the iodine film. This is 

 probably the cause of the discrepancy in height between the two 

 curves in the figure. It should be remembered that the galvanometer 



