distinguish still more colors which cannot be seen at all by the human eye. In general, names have not been given these invisible colors; they are described by their wave lengths, which are of the order of magnitude of two ten-thousandths of a millimetre. Each of these kinds of ultraviolet has its own characteristic properties and is as different from the other kinds as blue is from green or red. This fact is of great importance and is one which is generally overlooked by those who speak of ultraviolet. Sadder to say, it is too often overlooked even by those who are attempting to make use of it. It is as essential to consider the color of the ultraviolet ray as it is to note whether the traffic tower light is red or green.

Ultraviolet colors being invisible, it is not always easy for the user to determine the composition of the radiation; it may be made up of one or many different kinds. He should, if in doubt, consult some authority and not make the common mistake of assuming that his ultraviolet will do the same thing as that of his physician or chemist. For example, certain of the sun's ultraviolet rays are responsible for the sunburn so readily obtained in the summer. These same rays experience considerable difficulty in passing through smoke and heavy layers of air, and therefore do not reach the earth's surface in such abundance in the winter. This fact, and not that it is cold, accounts for the slowness with which one receives a sunburn in winter. At high elevations, as on snow capped mountain tops, sunburn is common. Exposure for hours or days to ultraviolet from which the burning rays have been excluded is attended by no discomfort.

Red glass is colored because it is unable to transmit the other colors of light. No known substance is colorless in the ultraviolet, but quartz and fluorite and a few other materials transmit a great many of the invisible colors and are therefore generally more valuable to the ultraviolet worker than are the glasses, no matter how colorless they appear to the eye; for the latter are, with few exceptions, opaque to most ultraviolet colors.

Often we see the statement that common window glass is opaque to ultraviolet. Such a statement is false. What should be said is that the glass is opaque to a few kinds of solar ultraviolet, and not that it does not transmit ultraviolet at all. As a

matter of fact it transmits a large percentage of the solar ultraviolet energy. The particular invisible colors which it does not transmit are believed to be of great therapeutic value, and, therefore, materials which are transparent to them have been developed to replace ordinary window glass. We find fused quartz, special glass and various flexible materials on the market, nearly all of which transmit to a certain extent the kinds of solar ultraviolet which cannot pass through ordinary windows. The choice depends upon how efficiently the material transmits the desired rays, and upon the cost. A wide variation in these factors is offered.

All very hot objects emit infrared, light and ultraviolet, the relative amounts of each depending upon the temperature. The sun, the filament of the modern electric incandescent lamp, and the crater of the carbon arc are good examples of such sources of ultraviolet. Flaming arcs, particularly the quartz mercury arc, and electric sparks, are powerful sources. It is well known that the visible colors of these various sources are decidedly different; they vary in appearance from red to blue and even white. Likewise, the invisible colors are far from being the same. The ultraviolet radiation from one source may not be able to do at all that which the radiation from another can accomplish.

The quantitative measurement of ultraviolet radiation is a fascinating art. The rays are directed upon a very thin, blackened strip of metal, where they are absorbed and converted into heat. The resulting minute increase in temperature of the strip is measured by means of a sensitive thermometer capable of detecting a few millionths of a degree rise. Such a thermometer is ordinarily an electrical device requiring extremely nice manipulation for its successful use. The instrument responds equally well to all colors of ultraviolet and can be employed in the study of visible and infrared radiation as well. There are other devices which respond to certain colors of the visible and invisible radiations, and may properly be employed under some circumstances, but for general work the method described is nearly always adopted.

While we attribute the discovery of ultraviolet to a Nineteenth Century scientist, the use of it in the treatment of disease is of

ancient origin. Sun treatments are known to have been given about 2000 years ago, and probably date back much further, because the practice of sun worship is very old. For various reasons sun treatments became a lost art until in the Eighteenth Century. Since that time they have enjoyed an increasing popularity.

The discovery that the ultraviolet in the sun's rays was largely responsible for many of the beneficial effects probably was one of the greatest single advancements made in heliotherapy. Immediately new fields were opened and much more careful scientific studies were made. Heliotherapy, the art, began to take on the aspects of a science.

It is not a full grown science yet, however, for much remains to be learned, and until there is closer coöperation between physicians and physicists there is little hope that it will become full grown. Neither the physician nor the physicist is sufficiently well informed in the other's field to permit him to undertake intelligent investigations in ultraviolet therapy. Probably in many instances one is unwilling to call upon the other for aid lest he lose credit to himself thereby. Fortunately, however, the need for united effort is being recognized to a greater and greater extent, so that within the next few years many facts concerning the use of ultraviolet in therapy should be brought to light.

At present we believe that the sun's ultraviolet is essential to our well being. This is not saying that the ultraviolet rays in sunlight are sufficient in themselves. Nature ordinarily does not work that way. Vitamins are essential to our diet, but who would dare to suggest that we try living on Vitamin A alone? Common sense seems to indicate that sunlight as a whole is essential to our well being. That ultraviolet is of great importance has been found by noting the change in health produced by removing certain kinds of this radiant energy from the sunlight by placing ordinary window glass between the subject and the sun. Furthermore, this is not saying that ultraviolet will cure every kind of disorder. There are indications that its rightful field is a large one, but in many cases it would appear far better to consider it as a necessary component of a balanced scheme. It may have its place along with correct diet and possibly medicaments in restoring general good health.

For years sanatoriums in different parts of the world have been relying in part on sunshine in the restoration of health, particularly in the treatment of pulmonary and surgical tuberculosis. Of course, it is always somewhat difficult to determine in so complicated a system as the human body just how much effect is due to sunlight and how much to carefully regulated diet and exercise. The results of the combination, however, are gratifying. The method of treatment with sunlight is interesting. The exposure begins with a small area of skin, often on the feet, and is of but a few minutes' duration. The following day the length of exposure to this same area is increased, and a short exposure is given to a new area. Progressively larger areas are exposed until the entire body can withstand direct sunlight for hours. Even in the winter tubercular children are permitted to play out of doors with shoes, short trunks and hats for clothing. They seem to enjoy their hikes, coasting parties and snow battles quite as much as the children who are bundled up to the ears.

Statistics show that rickets are most pronounced in the spring in the case of children who have been kept indoors the greater part of the time during the winter months, and that after playing out in the sunshine during the summer the disease is not nearly so prevalent. Furthermore, children with rickets can be cured through exposure either to direct sunlight or to artificial ultraviolet of the proper composition. These facts seem to indicate that ultraviolet has its place in the maintenance of good health.

Whatever the minute facts concerning ultraviolet therapy may be, the present tendency is to expose oneself to sunlight as much as possible, both on the beach and in the home or office or school. The majority of the medical profession agrees that this is wise. Many schools are replacing their old windows with panes of special glass which do not entirely obstruct the passage of those colors of ultraviolet which are known to be essential, and the time will undoubtedly come when nearly all of us will have at least a few "ultraviolet windows" in our homes and offices. Sedans, busses, street cars and railway cars will likewise follow suit. Possibly some progressive bus company will soon advertise that its patrons can obtain their daily sun baths while riding to and from work; it should appeal to the public.

The subject of special windows requires some further discussion, however. In the first place, in order to obtain the beneficial effects of solar ultraviolet the radiation must strike the body. It is not always possible nor is it ordinarily desirable to sit in the direct path of glaring sunshine. Yet those who are not in fairly bright sunlight may not expect to profit much by the expensive window installation. Use needs to be made of two characteristics of ultraviolet, namely, that it can be scattered by a diffusing medium and that it can to a certain extent be reflected by many kinds of surfaces, just as visible light is reflected and scattered. If a room be equipped with windows which will effectively diffuse the radiation and be painted with a paint having high reflectivity for ultraviolet, it no longer remains necessary to sit in the path of the sunlight. Diffusing skylights should be nearly ideal.

In the second place it is conceivable that a noticeable increase in the rate of fading of our rugs and draperies will result from their exposure to the shorter wave-length rays which we now exclude through the use of ordinary glass. It is known that these rays are active destroyers of some of the dyes. How serious this menace will prove to be we cannot at present predict.

The length of exposure to ultraviolet required to maintain or restore good health appears to be governed by the pigment of the skin. Physicians irradiating babies having rickets have noted that exposures which were sufficient for the White ones were entirely inadequate for their Negro cousins. A similar, although perhaps less marked, difference exists between blondes and brunettes. It is the experience of many who overexpose tender skin to the summer sun that more or less painful sunburn results, whereas if short exposures are taken at first, a coat of tan is developed which effectively reduces or removes the danger of becoming burned. The same thing is experienced by those who expose themselves to powerful electric arc radiation. Some do not tan to as dark a shade under the arc as under the sun, but the resistance to burning increases. It is the belief of many that tan also decreases the desirable therapeutic action of ultraviolet, so that longer exposures are required to accomplish what shorter ones could do before the tan appeared. Exactly what rôle tan plays here seems not to be clearly understood.