This section is from the "A Practical Treatise On Materia Medica And Therapeutics" book, by Roberts Bartholow. Also available from Amazon: A Practical Treatise On Materia Medica And Therapeutics
The rays of the sun may be applied directly to the whole surface, or to any part of the body. When the whole body is exposed to the rays of the summer sun, the head and face ought to be protected. The skin becomes warmer, the capillaries dilate, an erythematous blush appears, and the amount of blood in the peripheral vessels is increased above the normal. The rays of the sun in midsummer, ranging from 95° Fahr. to 125° Fahr., produce considerable burning heat, and cause a superficial inflammation of the skin, which is followed by desquamation. When the heat is less powerful, only an agreeable sensation of warmth may be experienced. Decidedly caustic effects may be produced by the concentration of the solar rays on a small spot of integument by a double-convex lens—a burning-glass, it has been called. This may be utilized as a means of counter-irritation.
Besides the heat, the solar rays contain chemical or actinic rays; and hence it is not improbable that chemical effects of a very important kind, or, it may be, catalytic effects, follow the application of the solar rays to a considerable portion of the body. Further, it can not be doubted that excitation of the cutaneous nerves by the heat and chemical rays of the sun must affect the condition of the brain and spinal cord, and, through the sympathetic system, the nutrition of the body.