SOME INTERACTIONS WITH LIVING MATTER 83 



within the crystal, the shorter wavelengths less so — and the image of the slit 

 will appear as darkening on a photographic plate, at positions proper to the 

 wavelengths entering the slit. Thus the absorption bands of water corre- 



O 

 spond to O — H stretching vibrations and to H H bending vibrations. 



This is true for any absorber with rotating or vibrating dipoles. Many 

 thousands of spectra have been determined, principally in organic mole- 

 cules, for purposes of learning what polar groups there are in the molecule, 

 or for identification of a particular substance in a mixture. Continuous use 

 is now being made of this technique in investigation and control of barbitu- 

 ates and narcotics, for example. Each material has a characteristic spec- 

 trum (plot of absorption vs wavelength), easily reproduced, in many cases 

 easily identified. Figure 4-6 shows two examples, and gives an indication 

 at the bottom of what rotations and vibrations within the molecule may be 

 responsible for each absorption peak (pointing down). 



Visible Radiations 



This region is noteworthy for the sole reason that the animal body is 

 equipped with a very sensitive set of living cells which can detect wave- 

 lengths of 4000 to 7800 A coming in from excited molecules in the environ- 

 ment. Molecules in the environment are excited by radiation which pours in 

 from the sun at all frequencies proper to a hot body. The reradiated energy 

 from the excited molecules of a tree, for example, outlines its shape; the 

 exact composition of the reradiated energy defines its brightness and what 

 we perceive as its color. 



The eye is a device by which the energy of an electromagnetic radiation 

 pattern is converted into the energy associated with the various nerve im- 

 pulses which can traverse the optic nerve to part of the brain. It is a trans- 

 ducer in the sense that it provides a mechanism by which electromagnetic 

 radiation of wavelengths in the critical range can be received, focused, sorted 

 out, and then converted into the chemical, thermal, and electrical energy 

 which is necessary to trigger nerve propagation. In general, the energy car- 

 ried by a nerve impulse is much greater than that of the light photons which 

 trigger the propagation. This subject is considered in Chapter 10, and we 

 confine ourselves here to what takes place before the nerve is triggered. 



Architecture of the Eye 



Figure 4-7 is a simplified sketch of the basic parts of the eye. It illustrates 

 principally the roles of the lens, the retina, and the optic nerve. Light of 

 intensity I Q ergs/cm 2 from a light source falls on the cornea. About 96 per 

 cent passes on through the lens, and about 4 per cent is reflected. The 

 cornea, the aqueous humor, the lens, and the vitreous humor are essentially 



