biophysics, application of various methods and principles of physical science to the study of biological problems. In physiological biophysics physical mechanisms have been used to explain such biological processes as the transmission of nerve impulses, the muscle contraction mechanism, and the visual mechanism. Theoretical biophysics tries to use mathematical and physical models to explain life processes. Radiation biophysics studies the response of organisms to various kinds of radiations. Biophysics has contributed important tools for the study of organic molecules, and especially of large molecules, which play an important part in biological processes. Paper chromatography, a direct development of adsorption techniques, is widely used to analyze tissues for chemical components. X-ray crystallography is used to determine molecular structures and has been useful with such problems as the complex structure of proteins. Among the optical methods used in the study of biological problems are photochemistry, light scattering, absorption spectroscopy (including the use of visible, ultraviolet, and infrared radiation), laser beams, and double refraction birefringence. These techniques and others permit the biophysicist to determine the structure of molecules in plants and animals to a degree not readily possible with ordinary chemical methods.