to Heat and Illuminaie Objects, 327 



tnelted between two white glafles, intercepted much light; 

 and when put on fufficiently thick, tranfmitted none. Many 

 difTereutlv coloured fluids that were alfo tried, I found were 

 hot fufficiently pure to be ufed, when denfe enough to ftop 

 light. Now red glafles, and the two laft-mentioned re- 

 fources of fmoke and pitch, any one of which, it has been 

 fcen, will ftop as much light as may be required, had ftill a 

 remaining trial to undergo, relating to diftin£tnefs; but this 

 I was convinced could only bf? decided by a6lual obfervations 

 of the fun. As an eafy way of fmoking glafles uniformly is 

 of fome confequence to di(tin6l vifion, it may be of fervice 

 here to give the proper dire<9:ions how to proceed in the ope- 

 ration: — With a pair of warm pliers, take hold of the glafs, 

 and place it over a candle at a fufficient diftancc not to con- 

 trail fmoke. When it is heated, but no more than ftill to 

 permit a finger to touch the edges of it, bring down the glafs 

 at the fide of the flame, as low as the wick will permit, which 

 nuift not be touched; then, with a quick vibratory motion, 

 agitate it in the flame from fide to fide j at the fame time 

 advancing and retiring it gently all the while. By this method 

 you may proceed to lay on fmoke to any required darknefs. 

 It ought to be viewed from time to time, not only to fee 

 whether it be fufliciently dark, but whether any inequality- 

 may be perceived J for if that fliould happen, it will not be 

 proper to go on. 



The fmoke of fealing-wax is bad ; that of pitch is worfe, 

 A wax candle sives a eood fmoke ; but that of a tallow candle 

 is better. As good as anv I have hitherto met with is tlt«f 

 fmoke of fpermaceti oil. In ufing a lamp, you may alfo have 

 the advantage of an even flame extended to any length. 



Telefcopic Experiments. 



No. I. By way of putting my theory to the trial, I ufecl 

 two red glalfes, and found that the heat which paffed through 

 them could not be fuHcred a moment; but I was now alfa 

 convinced that diftiniSlnefs of vifion is capitally injured by 

 the colouring matter of thefe glaflts. 



No. 2. I fmokcd a white glafs till it flopped light enougli 

 to permit the eye to bear the fun. Thisdellroyed »li diftintit- 



ncfs ; 



