44 MAMMALIA— MAN. 



" This was performed on the 20th of April, at eleven o'clock in the fore- 

 noon. I was alone with the surgeon, but during the operation the tooto. 

 swarmed with human forms of every description, which crowded fast one 

 on another ; this continued till half past four o'clock, exactly the time when 

 the digestion commences. I then observed that the figures began to move 

 more slowly ; soon afterwards the colors became gradually paler, and every 

 seven minutes they lost more and more of their intensity, without any 

 alteration in the distinct figure of the apparitions. At about, half past six 

 o'clock, all the figures were entirely white, and moved very little ; yet the 

 forms appeared perfectly distinct ; by degrees they became visibly less plain, 

 without decreasing in number, as had often formerly been the case. The 

 figures did not move off, neither did they vanish, which also had usually 

 happened on other occasions. In this instance they dissolved immediately 

 into air ; of some, even whole pieces remained for a length of time, which 

 also by degrees were lost to the eye. At about eight o'clock, there did not 

 remain a vestige of any of them, and I never since experienced any appear- 

 ance of the same kind. Twice or thrice since that time, I have felt a pro- 

 pensity, if I may be so allowed to express myself, of a sensation, as if I saw 

 something which in a moment again was gone. I was even surprised by 

 this sensation, whilst writing the present account, having, in order to render 

 it more accurate, perused the papers of 1791, and recalled to my memory 

 all the circumstances of that time. So little are we sometimes, even in the 

 greatest composure of mind, masters of our imagination." 



As the sense of sight is the effect of the action of light upon the eye, it is 

 well kno-\vn that too much light as well as too little, is extremely prejudi- 

 cial. Travellers, Avho cross countries covered with snow, are obliged to 

 wear a crape before. their eyes. Persons, therefore, who read or AVTite much, 

 should accustom themselves to a moderate light. 



There are many reasons to induce us to suppose, that such persons as are 

 short-sighted, would see objects larger than others ; and yet it is a certain 

 truth that they see them smaller. 



Error is however not confined to any one sense ; and that of hearing is 

 liable to similar mistakes with that of sight. This sense conveys no distinct 

 intelligence of the distance whence a sounding body is heard : a great noise 

 far ofi", and a small one very near, produce the same sensation ; and, unless 

 we receive information 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 when we hear it. When, for example, we know the tone 

 of a bell, we are then at no great loss to determine how far it is from us. 



The air is the principal means of conveying the sound.* Sound is in 



* The strokes of a bell give no sound, when it is placed under the receiver of an air- 

 pump, which is exhausted of its air. 



