Cases of this sort are comparatively rare in civil life, the conditions which lead to them being usually wanting; for it will be noticed that even the military officer takes the greatest care not to be “smart” when he goes out shooting under a tropical sun, his costume, when so engaged, being generally a model of the way he and his men ought to be equipped when engaged in the business of their profession. What may be called true sunstroke is less common, and beyond the bare fact that the sun’s rays are capable of acting in this way, we are really quite without any explanation of its causation, as the condition may be brought about by comparatively short exposure, without any coincident exhaustion or fatigue; and may occur at times when the temperature of the atmosphere is by no means excessive. It is probable that over-stimulation of the nerves of sight by glare may have something to say in the matter, as it has been found that persons at rest in the open, in the Tropics, have their temperature less raised if they wear darkly tinted spectacles than others similarly situated, but having the eyes unprotected. This cannot, however, be the complete explanation, as it appears to be exposure, not of the face, but of the skull and back-bone, to the direct rays of the sun that constitutes the real danger, and it is not the forehead but the back of the head and temples that are most sensitive to the influence of “insolation.” So many unexpected discoveries have been made lately of forms of light whose very existence was, but a little time ago, unsuspected, that the matter is less inexplicable than it was before the discovery of the Röntgen rays made us familiar with light vibrations capable of passing easily through substances we have been accustomed to regard as quite opaque. The rays of the sun do not, of course, include vibrations of that particular description, or none of our ordinary wooden photographic apparatus would be of any use to us; but they may well have amongst them other vibrations, as yet not identified, which are capable of passing through the tissues and affecting the brain and spinal cord beneath; and no other suggestion appears capable of explaining the extraordinary way in which a few instants of exposure of the unprotected head to an Indian sun suffices to cause a sharp headache; as the effect is utterly inexplicable on any mere assumption of rise of temperature. It is quite a mistake, for example, to think that it is safe to cross one’s garden hatless, from the house to one’s stables, in the heat of an Indian day. One is not of course likely to be stretched out in a state of insensibility by so short an exposure, but you may easily earn a splitting headache which will last you the rest of the day; and from the time the sun is well over the horizon, till it again sinks beneath it, it is a purposeless imprudence to be found in the open with the head inadequately protected.