Thermodynamics, as the name implies, is involved whenever there is a transfer into heat energy of any other form of energy, or vice versa; and for the investigation and study of such phenomena there have been developed cer tain general relationships, theorems and methods of treatment, which, taken together in their proper arrangement, form a peculiar and dis tinct mathematical branch of science which we may designate as °formal" thermodynamics, to distinguish it from the broader field of gen eral thermodynamics. The applications of this thermodynamic reasoning are found in every branch of physical and chemical theory, includ ing electrical, electrochemical and radiation theory.
The best-known of the relationships of °formal° thermodynamics are the Carnot theorem and the Kelvin or thermodynamic tem perature scale. The latter furnishes a tem perature standard which is independent of the physical properties of any physical substance, while the two together not only furnish a cri terion for the efficient performance of any heat engine (steam, gas, etc.), the purpose for which they were originally developed, but are also the basis of relationships connecting the factors involved in change of state (e.g., the Clapeyron Clausius equation), thermoelectricity, solution and innumerable other physical and physico chemical phenomena. Certain special thermo dynamic functions such as entropy, which is the common one, have been formulated to simplify the mathematical treatment of these problems. General thermodynamics includes, in addition to the reasoning of °formal° thermo dynamics and the applications mentioned above, a thorough study of the relations connecting the mechanical and thermal conditions of matter in the different states, and in the transition states: such as, for example, the relation of pressure and specific volume to the temperature, specific heats, heat of transformation, etc., and their interpretation according to thermodynamic reasoning. The idea of a ((perfect" gas, and the evolution of the kinetic or dynamic (mole cular) theory of gases, are logical developments of this section of physics. In recent years the methods of °statistical° mechanics which have been developed for the study of certain types of thermodynamic problems have met with con siderable success in the study of the °dynamic° theory and of radiation theory. The many ap plications of thermodynamic theory and rea soning in heat-power engineering, physical chemistry and other fields, might properly be classed as applied or technical thermodynamics.
Light is a form of radiant energy capable of stimulating the nerve termini of the retina of the human eye. Two well-defined methods of study are generally recognized. To a limited extent, its phenomena may be studied, and simple laws for its reflection and refraction by mirrors, prisms and lenses be deduced, without reference to the physical nature of light energy, or of its propagation. This is the method of 'geometrical optics, as distinguished from phys ical optics; and it suffices for the design of cameras, telescopes, surveying instruments, range finders, etc., and for most of the work of perfecting the optical surfaces of the lenses used in such instruments (to make them achro matic, anastigmatic, aplanatic, etc.), although intelligent work in this field is now guided by a knowledge of many of the phenomena which come within the scope of physical optics. In physical optics, the phenomena of light are studied for the purpose of determining the nature of light energy and of its propagation, and of its relation to other forms of energy. Lately this has extended to the study of the nature of the bodies that produce light, and as well, of those that reflect, refract or otherwise modify its character or propagation. Study of the phenomena of interference, of diffraction, of transmission in finite time, and of polariza tion, established the theory (the wave theory) that light energy is propagated through space as a transverse wave motion at a finite, though incredibly high, velocity. Spectral color is identified with wave length. Later investiga tions have identified light radiation as that part of electromagnetic radiation whose wave lengths are capable of exciting the visual sense. The wave theory of light gives a clear and logical explanation of all of the phenomena of reflection and refraction, and in addition, has disclosed physical limitations to the clearness of the optical images produced thereby, other than the limitations to the perfection of the optical surfaces, the knowledge of which has done much toward the perfection of such optical instruments. Furthermore, the development of apparatus for the demonstration and study of the phenomena peculiar to physical optics has resulted in the invention of many new and valu able physical instruments — interferometers, diffraction gratings, polarimeters, etc.— which open up new fields of knowledge and study, and permit of the measurement of many physi cal quantities, both new and old, to a previously unheard-of degree of accuracy.