4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 87 



wave-length range in the transmitted beam. Unfortunately the few 

 filters which are available either transmit too wide a band for sig- 

 nificant experimentation or transmit too low an intensity for practical 

 purposes. Consequently lamps of the incandescent type together with 

 available filters fail to meet a very urgent need for practically mono- 

 chromatic sources of illumination. 



This need is met by the last mentioned group, namely, electrical dis- 

 charge through gases. Such sources of illumination have been in 

 common laboratory use for physical investigation for decades. Com- 

 mercial application however, until very recently has been limited to 

 the mercury arcs. The development of Neon signs has stimulated in- 

 dustrial interest in such sources of light which emit selectively a limited 

 number of practically monochromatic " lines " of radiation. Helium 

 tubes are now available which furnish an intense yellow radiation and 

 *sodium tubes have been developed and should be available in the rela- 

 tively near future. Hydrogen tubes are a matter of common laboratory 

 practice. These yield a very strong red line together with less intense 

 blue-green and blue lines. 



In order that these sources may be used intelligently it is necessary 

 that the distribution of energy of a given source maintained under 

 specified conditions be known. While data are available so far as 

 the common incandescent light is concerned, such is by no means the 

 case in regard to the sources of selective emission, namely, discharges 

 through gases. In order to meet this situation our laboratory has been 

 equipped not only to make such special lamps as are not available, 

 but also to make photometric determinations of the distribution of 

 intensity in terms of well-defined standards for all types of sources. 

 Since information is required for the entire range from i.5ju, to .2fx, 

 both as to emission of sources and transmission of windows, the spec- 

 troscopic developments are rather unusual in scope. Automatically 

 recording instruments are being developed which will yield absolute 

 intensities for the entire range. Results of these developments will 

 be discussed in other papers. 



PLANT GROWTH CHAMBERS 



With the general attributes of available sources of radiation in mind, 

 a set of growth chambers has been developed whose primary purpose 

 is to expose plants to different radiation conditions while Qther factors 

 of environment are maintained essentially the same. In order that 

 temperature, humidity, and gas concentrations may be as nearly 

 identical as possible, the set of four chambers is controlled by a central 

 reservoir and manifolding system. Both air and water are distributed 

 from common sources, for the sake of temperature control. By the 



