SOME INTERACTIONS WITH LIVING MATTER 91 



small, the condition is called hypermetropia, and can be corrected with a con- 

 vex lens of proper focal length. 



The lens of the eye often does not have the same curvature over all its sur- 

 face, and light passing through the area of improper curvature will not be 

 properly focused on the retina. The lens of such an eye is said to be astig- 

 matic. A properly ground astigmatic glass lens can compensate. 



Sometimes translucent or opaque tissue grows in or on the liquid crystal 

 material of the lens and absorbs the incoming light before it reaches the 

 retina. Such tissues are generally termed cataracts. Some can be removed by 

 surgery; some are too extensive. 



Depth Perception 



Two detectors in different locations can inherently provide more informa- 

 tion than one; and if relative information is recorded and interpreted from 

 the two signals, more information is available from the two detectors than if 

 each were interpreted separately. This is the reason sensory organs come in 

 pairs. Typical of the relative information obtainable from two stations, in 

 general, are direction and distance, or depth. Sound can be reflected, and 

 hence the directional information provided by two stations is important. 

 Light travels in a straight line to the eye, and therefore directional informa- 

 tion is not important. However, the information derivable about distance or 

 depth is important when we attempt to compare distances or develop a per- 

 spective view. Ideally the eyes may each be rotated about 50° from a central 

 line of vision. The two have to be in focus at the same time, on a near or a 

 far object, and this requires a facility of minor individual adjustment. If the 

 eyes cannot be made to focus (crossed eyes), sufficient correction can some- 

 times be made with a suitable set of glass lenses, but often the cross must be 

 corrected by shortening the lateral muscles or by suitable exercises designed 

 to strengthen them. 



Photochemical Radiations (Ultraviolet) 



Photosynthesis 



Subshell electrons are excited by the ultraviolet. The absorbed energy 

 may be passed off to the vibrations or rotations of nearby molecules and ap- 

 pear as heat energy; it may be re-emitted as ultraviolet; or it may excite 

 the molecule and make it more susceptible to chemical attack by neighbor- 

 ing molecules. Thus in the last case the ultraviolet may provide some or all 

 of the activation energy needed for reaction to occur, and thereby increase 

 the rate of reaction (treated later in Chapter 8). In fact, the photochemical 

 mechanism is sometimes the only mechanism by which certain reactions can 

 take place at a reasonable speed at biological temperature. 



