270 OPTICS. 



fire and mirror, he will will see an image of the 

 fire upon the table, large and erect. And if another 

 person, who knows nothing of this matter before- 

 hand, should chance to come into the room, and 

 should look from the fire towards the table, he 

 would be startled at the appearance ; for the table 

 would seem to be on fire. In this experiment 

 there should be no light in the room, but what 

 proceeds from the fire, and the mirror ought to be 

 at least fifteen inches in diameter. 



Take a glass bottle, fill it partly with water, and 

 cork it in the common manner: place this bottle 

 opposite a concave mirror, and beyond its focus, 

 that it may appear reversed; then place yourself 

 still farther distant than the bottle, and it will be 

 seen in the air inverted, and the water which is 

 actually in the lower part of the bottle, will appear 

 to be in the upper. 



If you invert the bottle whilst before the mirror, 

 the image of the water will appear in the lower 

 part of the bottle ; when it is in this inverted state, 

 uncork the bottle, and whilst the water is running 

 out, the image is filling; but as soon as the bottle 

 is empty, the illusion ceases. If the bottle like- 

 wise be quite full, there is no illusion. The re- 

 markable circumstances in this experiment are : 

 1. Not only to see an object where it is not, but 

 also where its image is not. 2. That of two ob- 

 jects, which are really in the same place, as the 

 surface of the bottle, and the water it contains, 

 the one is seen in one place, the other in another. 

 It is supposed, that this illusion arises partly from 

 our not being accustomed to see water suspended 

 in a bottle with the neck downwards, and partly 

 from the resemblance there is between the colour 

 of air and the water. 



