In the case of the eye, the parts of the visible spectrum evoke different sensations of colour, viz., red, orange, yellow, blue, violet, as the wave lengths shorten from 8,000 to 4,000 A.U. The living substance of the retina has evolved so as to react differently to the different groups of wave lengths. The living cells of the skin and the blood in the skin also react differently to infra-red, visible and ultra-violet rays, but little is yet known about this subject. The U.V. rays are shorter than violet ones, and they, therefore, lie beyond the violet of the spectrum and are invisible, but may be made visible by fluorescent screens.
The horny layer of the skin fluoresces faintly ; it can be made to fluoresce strongly by a coat of vaseline or solution of quinine. Fluorescence is produced by electrons displaced from atoms by the rays jumping back into inner orbits. A part of the energy of the rays is thus spent ; fluorescence of the surface of the skin is then a means of protection against U.V. rays.
For study of the U.V. rays a quartz spectroscope is required, as glass absorbs most of these rays. They are classified by wave length as "near," 4,000 to 3,00o A.U., and "far," 3,000-2,000 A.U. The region about 3,100-2,900 A.U. is sometimes called the middle U.V., or "Dorno region," and has the most powerful sunburn action on the skin. These middle U.V. rays are so completely absorbed by living substance that they have very little power of penetration; thus, only some i o 0 of rays, about 3,00o A.U., penetrates as far as the capillaries beneath the epidermis.
The main action of these rays is then, on the living cells of the epidermis. The U.V. rays shorter than 2,500 A.U. are so com pletely absorbed by the outer horny layer of the epidermis that they have very little action on the living cells beneath ; the very far U.V. rays less than 1,800 A.U. do not penetrate air. A powerful source of U.V. has a bluish white light, and produces a pricking sensation in the conjunctiva of the eyes. To prevent conjunc tivitis the eyes must be protected by glasses from such sources.
The skin is insensitive to these rays on exposure, but after suffi cient exposure and a latent period of a few hours, there follow in flammatory reactions of the skin ; these are pricking, flushing and slight swelling, followed later by peeling and browning. The reac tion is caused primarily by the rays displacing electrons in the atoms of certain substances in the living cells, and so inducing molecular changes which after a latent period result in coagulation and death of the living substance. The dead cells desquamate and are replaced by new ones. Products of the damaged cells, by ex
citing the nerve endings in the epidermis, cause flushing of the blood vessels of the skin, and transudation of lymph and leuco cytes. These local reactions in the irradiated skin provoke sec ondary reactions in the blood and body generally.
The effect of U.V. rays on living cells can be studied under the microscope, e.g., on infusoria enclosed in a suitable quartz cham ber. With the help of a quartz microscope and photography the effect of U.V. rays on microbes has been studied. Such a method shows up structures in living micro-organisms previously unknown. The remarkable discovery has been made that a substance, ergo sterol, which is present in foods and in the skin, is activated by middle U.V. rays and becomes vitamin D required for bone forma tion. The want of this vitamin causes rickets and softening of the hones. Rickets can be prevented by adequate exposure of the skin to the U.V. rays of the sun or arc lamps, or by taking enough vitamin D in the food.
This one fact by itself shows the immense importance of getting rid of smoke pollution of the air, and having sun baths. Walls, glass, smoke, fogs and clothes cut off people from these beneficent rays. Irradiation of rickety children with U.V. rays increases the percentage of inorganic phosphorus and calcium in the blood, which is in them abnormally low. While provoking the formation of the brown pigment (melanin) in the skin, the U.V. rays lessen at the same time the percentage of the amino-acid tyrosin in the blood, the probable precursor of melanin. The inflammatory reac tion set up by the U.V. rays provokes an increase of the power of the blood to destroy microbes as tested in vitro, a sign that these rays may possibly increase resistance to infection. It must be borne in mind that an over-dose has the opposite effect.
Right dosage with U.V. rays increases the sense of well-being and alertness of mind, and acts as a tonic in the winter months. Much of the stimulating effect of a visit to the Alps can be imi tated by arc-light baths, combined with car rides and exercise in the open air. The fact must not be lost sight of that exposure to cool open air is as important as exposure to light. Such exposure stimulates the heat production of the body and appetite and tones up the muscles; breathing cool open air provokes blood flow through, and secretion from, the respiratory membranes ; these are excellent effects. Open air is clean compared with house air which is contaminated with dust, mould and microbes.