When an object is extremely brilliant, or we fix our eyes too long upon the same object, the organ is hurt and fatigued, vision becomes indistinct, and the image of the object, having made too violent an impression, appears painted on every thing we look at, and mixes with every object that occurs. How dangerous the looking upon bright and luminous objects is to the sight, is evident from the effect it has on the inhabitants of countries which are covered for the greatest part of the year with snow; and travellers, who cross those countries, are obliged to cover their eyes with crape. In the sandy plains of Africa, the reflection of the light is so strong, that it is impossible for the eye to sustain the effects of it. Such persons therefore, as write, or read for any continuance, should chase a moderate light, for though it may seem insufficient at first, yet the eye will gradually become accustomed to the shade; and at any rate, it will be less injured by too little light than by too much. CHAPTER VII. OF THE SENSE OF HEARING. As the sense of hearing, as well as that of seeing, gives us perceptions of remote objects, so it is subject to similar errors, and may deceive us, when we cannot rectify by the touch, the ideas which it excites. It communicates no distinct intelligence of the distance from whence a sounding body is heard: a great noise far off, and a small one near, produce the same sensation, and, unless we receive aid from some other sense, we can never distinctly tell whether the sound be a great or a small one. It is not till we have, by experience, become acquainted with any particular sound that we can judge of the distance from whence we hear it; but if, for example, we hear the sound of a bell, we are at no great loss to determine its distance, any more than we are of that of a cannon from the report, judging in both cases from similar sounds, which we have been previously acquainted with. Every body that strikes against another produces a sound which is simple in bodies non-elastic, but is often repeated in such as are elastic. If we strike a bell, a single blow produces a sound, which is repeated while the sonorous body continues to vibrate. These undulations succeed each other so fast, that the ear supposes them one continued sound; whereas, in reality, they form many. A circumstance of this kind happened to myself, for lying on the bed half asleep, I distinctly counted five strokes of the hammer upon the bell of the clock, and rising immediately found it was but the hour of one, and was convinced by examining the machinery that it had struck no more. A person, therefore, who should for the first time, hear the toll of a bell, would very probably be able to distinguish these breaks of sound; and, in fact, we can readily ourselves perceive remission in sounds.