Through the dawn of day and the fall of night and the passing of the seasons, primitive man first became acquainted with the association between light and heat. It was brought home still more dramatically through his own artifices when he learned to make fire by rubbing together two pieces of wood, for in this manner he was provided with a luminous flame by which to see and a means by which to cook his food and warm himself. In all stages of civilization man has recognized the benefits of light and heat and the fact that the means employed to produce the one generally gives rise to the other. In the fire of Aristotle they were combined as one of the basic elements of the physical world. Even the burning glass was known to the ancients, for we find that they used glass globes filled with water to concentrate the rays of the sun and so produce fire. From those early days up to the present time, as science became more and more grounded on a firmer basis, man's knowledge of the behavior and nature of these associated principles of light and heat grew. He now knows them to be closely related phenomena in the realm of the concept which the physicist calls radiation. From the early observations on the behavior of light and heat this science of radiation has grown, and from it most of our knowledge of the nature of matter has been derived.
The subject of this book is the photography of those radiations which are known as the "infrared." It will be shown a little later that these are identical with the rays of heat, and they are, therefore, associated with light. In fact, in order to obtain a clear understanding of the subject, we must first of all study some of the characteristics of the light which we can see, and then extend our interest to its close relative, the infrared. This is important, not only for an insight into the nature of the infrared, but also because most photographs are made by visible light,