140 • 'Os Optics.

double the distance made less by the radius, and the quotient is thedistance required.

If the object be in the Center of the mirrour’s concavity, the imageand object will be coincident, and equal in bulk.

If a man places himself directly befcre a large concave mirrour,but farther from it than its Center of concavity, he will sce an invertedimage of himself in the air, between him and the mirrour, of a less sizethan himself. And if he holds out bis band towards the mirrour, theband of the image will come out towards bis band', and coincide withit, of an equal bulk, when his band is in the Center of concavity; andhe will imagine he may fhake hands with his image. If he reaches hisband farther, the band of the image will pafs by his band, and comebetween his band and his body : and if he moves his band towards ei-ther side, the band of the image will move towards the other; so thatwhatever way the object moves, the image will move the contrary.

All the while a by-standet* will fee nothing of the image, becaufenone of the reflected rays that form it enter his eyes.

If a fire be made in a large room, and a smooth mahogony tablebe placed at a good distance near the wall, before a large concave mir-rour, so placed, that the light of the füre may be reflected from themirrour to its focus upon the table; if a perfon stands by the table, hewill sce nothing upon it but a longifh beam of light: but if he standsat a distance towards the fire, not directly between the fire and mir-rour, he will sce an image of the fire upon the table, large and erect.And if an other perfon, who knows nothing of this matter before-hand, fhould cbance to come into the room, and fhould look fromthe fire towards the table, he would be startled at the appearance ; forthe table would seem to be on fire, and by being near the wainscot, toendanger the whole house. In this experiment, there fhould be no lightin the room but what proceeds from the fire; and the mirrour oughtto be at least fifteen inches in diameter.

If the fire be darkened by a fcreen, and a large candle be placed atthe back of the fcreen; a perfon standing by the candle will sce theappearance of a fine large star, or rather planet, upon the table as brightas Venus or Jupiter. And if a fmall wax taper (whose flame is muchless than the flame of the candle) be placed near the candle, a satelliteto the planet will appear on the table; and if the taper be movedround the candle, the satellite will go round the planet.

For these two pleafing experiments, I am indebted to the reverendDr. Long, Lowndes ’s profestor of astronomy at Cambridge, who fa-voured me with the fight of them, and many more of his curious in-ventions. In