LIGHT AND PL.'VNT GROWTH BRACKETT AND JOHNSTON 263 



stations coniinf; in :it once. The construction of such an infru-retl 

 rccoivin<r equipment or spectrometer which will automatically record 

 the intensities of ratliation of dillerent wave len<,'ths ui)on a moving 

 photographic plate is a part of the program. One such instrument 

 to he used in the ran<i:e of wave lengths between l/i and 2.5/i or 

 between 1 and 25 millionths of a meter is already completed and in 

 operation. Otiier machines to operate in the region between 2/i and 

 15/1 are under construction. 



u\ ^^j^^Lk^ 



LOO 



y 



1 10 



l.ZO 



1.30 



U 



1.40 



7 



1.60 



I.GO 



170 



1.80 



FiGLKB 1. — Emission spectrum of tbc mercury arc 



Molecules not only emit frequencies which are characteristic of 

 them, but also when light is passed through them, absorb through 

 resonance the same frequencies. Often it is more convenient to study 

 their absorption than their emission. Thus light may be taken 

 from an ordinary tungsten Mazda bulb, passed through a liquid, 

 the structure of whose molecules is to be studied, and analyzed to 

 see what frequencies have been weakened. The top curve of Figure 

 2 shows the light from a Mazda bulb passed through 1 cm. of carbon 



