256 FERGUSON'S LECTURES. 



LECT. All the while a by-stander will see nothing of the 

 T^L, image, because none of the reflected rays that form it 

 enter his eyes. 



If a fire be made in a large room, and a smooth ma- 

 hogany table be placed at a good distance near the wall, 

 before a large concave mirror, so placed that the light 

 of the fire may be reflected from the mirror to its focus 

 upon the table ; if a person stands by the table, he will 

 see nothing upon it but a longish beam of light : but if 

 he stands at a distance towards the fire, not directly be- 

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

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

 person, who knows nothing of this matter beforehand, 

 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, 

 and by being near the wainscot to endanger the whole 

 house. 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. 



If the fire be darkened by a screen, and a large can- 

 dle be placed at the back of the screen ; a person stand- 

 ing by the candle will see the appearance of a fine large 

 star, or rather planet, upon the table, as bright as Venus 

 or Jupiter. And if a small wax taper (whose flame is 

 much less than the flame of the candle) be placed near 

 the candle, a satellite to the planet will appear on the 

 table : and if the taper be moved round the candle, the 

 satellite will go round the planet. 



For these two pleasing experiments,'! am indebted to 

 the late Rev. Dr. LONG, Lowndes's profess or of astronomy 

 at Cambridge, who favoured me with the sight of them, 

 and many more of his curious inventions. 



In a refracting telescope, the glass which is nearest the 

 object in viewing it, is called the object-glass ; and that 



minated by a strong light, while the mirror is in darkness, the dagger 

 appears directed towards the heart of him who wields it. 



