422 HAROLD F. BLUM 



by the human eye — wavelength approximately 390 to 650 m^; {2) the 

 ultraviolet, that region of wavelengths shorter than the visible and ex- 

 tending down to the longer X-rays; (3) the infrared, that region longer 

 than the visible and extending as far as the shortest radio wavelengths. 

 The division into visible, ultraviolet, and infrared is arbitrary, since 

 the sensitivity of the human eye does not end abmptly at the points 

 assigned (see Figs. 3 and 4), and these divisions of the spectrum have 

 no true physical or biological meaning. Since room air absorbs virtu- 

 ally all ultraviolet radiation shorter than about 180 mju and infrared 

 radiation longer than about 30 m to such an extent that special meth- 

 ods of working are required, and also because the absorption by living 

 organisms themselves sets an even narrower range (see 3) most meas- 

 urements on biological systems are limited to these extremes. There 

 are other reasons for limiting the range of biological studies. Wave- 

 lengths lying in the infrared beyond about 1 /x represent small quanta, 

 which correspond only to the quantized states of rotation and vibra- 

 tion of the atoms in the molecule, changes in electronic energy level 

 requiring larger quanta. Longer wavelengths than this produce no 

 photochemical or photobiological action, because excitation of the 

 molecule that will permit it to take part in chemical reaction requires 

 such a transition in electronic energy level. Thus, so far as biological 

 action spectra are concerned, there is a wavelength limit, somewhere 

 in the near infrared, beyond which no such effects are to be expected. 

 The more distant region of the infrared in which only rotation and 

 vibration spectra are represented is of interest in another way, because 

 such spectra represent intramolecular forces characteristic of the 

 structure of the molecule. Their exploration with modern instru- 

 ments is becoming more and more useful in the study of the structure 

 of organic compounds, but this is a subject outside the scope of this 

 chapter (see 5,8). 



1. Quantitative Measurement of Absorption Spectra 



The complete determination of an absorption spectrum requires 

 measurement of the relative extent of absorption at various wave- 

 lengths. Methods of obtaining and of measuring monochromatic 

 radiation are treated very briefly in Section CI ; here only the meas- 

 urement of absorption of monochromatic radiation is discussed. 



Absorption Laws and Units. It is essential to consider first the 

 so-called absori^ion laws. The intensity of a parallel beam of 



