In darkness and solitude, when surrounding objects produce no images that can interfere with those of the mind, these latter are more lively and distinct: and when in addition we are half asleep and half awake, the intensity of mental impressions approaches that of visible objects. In the case of persons of studious habits who are continually employed in mental effort, these images are more distinct than with those who follow the ordinary avocations of life, and during their working hours rarely see the objects round them. The earnest thinker, absorbed by meditation, is in a manner deprived for the time of the use of his senses. His children and servants pass in and out of his study without his seeing them, they speak to him without his hearing them and they may even try to rouse him from his reverie without success; and yet his eyes, ears, and nerves received the impression of light, sound, and touch. In such instances, the mind of the philosopher is voluntarily occupied in following out an idea which interests him profoundly; but even the most unlearned and thoughtless of us sees the images of dead or absent friends with his mind’s eye, or even fantastic figures which have nothing to do with the train of thought he may be pursuing. It is with these involuntary apparitions as with spectres of the imagination: although they are intimately connected with some thought that has passed through our mind unperceived, it is impossible to trace a single link of the chain connecting them together. ------------------------------------------------------------------------ PART II. THE LAWS OF LIGHT. -------------- CHAPTER I. WHAT IS LIGHT? EVERYBODY knows the effects of the action of light, without, however, understanding precisely what constitutes light itself. Any formal definition would rather puzzle than help the student; we must therefore content ourselves with saying that light is that effect of force which causes us to perceive external objects. A man who was blind from his birth, and upon whom the operation for cataract had been successfully performed, had accustomed himself for a long time to imagine the nature of those unknown phenomena that his affliction had prevented him from observing. He had arranged in his mind the various definitions that had been given to him as to the nature of light, and having combined them, he fancied he had acquired some notion of what the sense of vision really meant. But what was the astonishment of the surgeon who had restored to him his fifth sense, when he asked him to give his opinion upon the effects of light, to see him take up a lump of sugar and reply that it was under that form that he had imagined it to himself.