NO. 13 FUNCTIONS OF RADIATION— BRACKETT AND JOHNSTON 3 



upon an automatically recording infra-red spectrograph constructed at 

 the Fixed Nitrogen Research Laboratory in cooperation with this 

 Division. For the visible and ultra-violet they have been determined 

 by means of a double monochromator using the special thermocouples 

 developed in this Division. Two standard Bausch & Lomb quartz 

 monochromators have been combined in order to secure greater wave- 

 length purity. These observations will be discussed in greater detad 

 in other publications. 



From a photochemical point of view this set of filters has a number 

 of interesting possibilities. Since photochemical reactions show a 

 long wave-length threshold, that is they do not proceed for any wave 

 lengths longer than a definite value (the threshold), it is possible with 

 such a group of filters to vary the illumination so that different photo- 



FiG. I. — Transmission curves of filters. 

 I Pvrex • 2 Nultra ; 3, Noviol, shade O ; 4, Noviol, shade C ; 5, Heat resisting 

 yellow! yellow shade; 6, Lighthouse red; 7, Heat res.stnig red, 212 per cent; 

 8 Heat resisting red, 122 per cent: 9, Heat resisting red, 62 per cent ; 10, Blue 

 purple ultra plul heai resisting red, 212 per cent; H, Heat absorbing Alko, light 

 shade. 



chemical reactions may be diiTerently afifected, provided they have 

 dififerent long wave-length thresholds. 



Another set of filters is also of interest in such experiments. This 

 is a group of heat-absorbing filters which remove the infra-red longer 

 than i/x and cut off in varying amounts the near infra-red and longer 

 wave lengths of visible radiation. Such filters are of particular im- 

 portance in view of the fact that standard incandescent lights pro- 

 duce an excessive amount of infra-red radiation as compared to the 

 sun At present only one type of heat-absorbing filter has been ob- 

 tained. The transmission of this filter is indicated by the dotted line H 

 in Figure i Other filters which transmit a greater proportion of light 

 in the red are being procured. These glass filters have the advantage 

 of a considerable degree of permanency. 



Only a few filters are available that eliminate both the long and short 

 wave-length ranges of the spectrum, leaving a definite band of limited 



