270 . ' PHILOSOPHICAL TRANSACTIONS. [aNNO 1668. 



sented, there must be placed a broad convex-glass, so that it may represent the 

 object distinct. The nearer it is placed to the object, the more is the object 

 magnified on the wall ; and the farther off the less ; which diversity is effected 

 by glasses of several spheres. If the object cannot be inverted, as it is pretty 

 difficult to do with living animals, candles, &c., then let two large glasses of 

 convenient spheres be placed at proper distances, to be found by trials, to make 

 the representations erect, as well as the object. 



These objects, reflecting and refracting glasses, and the whole apparatus, as 

 also the persons employed to manage them, must be placed without the window 

 or hole, so that they iliay not be perceived by the spectators in the room. 



Whatsoever may be done by means of the sun-beams in the day time, the 

 same may be done with much more ease in the night, by the help of torches, 

 lamps, or other strong lights placed about the objects, according to the several 

 sorts of them. 



So far our inventor; who has not contented himself with the bare speculation, 

 but put the same in practice some years since, in the presence of several mem- 

 bers of the Royal Society. 



Of Counterfeiting Opal, and making lied Glass. By Mr. S, Cole- 



PRESSE. N" 38, p. 743. 



I was two days at Haarlem, on purpose to see the experiment of the making 

 of counterfeited opal glass, which is there done by rule. It is very lively, and, 

 as I guess, performed only by the degrees of heat producing the colours. 

 When the composition is thoroughly melted, they take out some on the point 

 of an iron-rod, which being cooled, either in the air or water, is colourless and 

 pellucid; but being put into the mouth of the furnace on the same rod, and 

 there turned by the hand for a little time, its little particles take such various 

 positions, that the light falling on them being variously modified, represents 

 those several colours that are seen in the true opal. It is remarkable that the 

 colours of it may be destroyed and restored again by different degrees of heat. 



They also make there the amethyst and sapphire ; and have recovered the 

 hundred years loss of incorporating red glass ; and have some metal that is 

 thought to equal crystal in hardness as well as colour. 



Some Animadversions, ivritten in a Letter by Dr. John Wallis, on a 

 printed Paper, entitled Responsio Francisci du Laurens ad Epistolam 

 B. Wallisii, ad CI. V. Oldenhurgium scriptam. iV" 38, p. 744. 



A continuation of the unprofitable controversy, noticed before in N° 34, 

 p. 239, between Dr. Wallis and M. du Laurens. 



