The following experiments are stated by Mr. Ferguson in his “Lectures on select Subjects,” &c. “If a fire be made in a large room, and a smooth mahogany 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 stand by the table, he will see nothing upon it but a longish beam of light: but if he stand at a distance toward the fire, not directly between 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 the matter beforehand should chance to come into the room, and should look from the fire toward 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 candle be placed at the back of the screen, a person standing by the candle will see the appearance of a very 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.” Many other illustrations of the effects of concave specula might have been given, but I shall conclude this department by briefly stating some of the _general properties of speculums_.