of the prismatic Colours to heat and illuminate Objects. 277 



consequence to distinct vision, it may be of service here to give 

 the proper directions, how to proceed in the operation. 



With a pair of warm pliers, take hold of the glass, and place 

 it over a candle, at a sufficient distance not to contract smoke. 

 When it is heated, but no more than still to permit a finger to 

 touch the edges of it, bring down the glass, at the side of the 

 flame, as low as the wick will permit, which must not be 

 touched. Then, with a quick vibratory motion, agitate it in the 

 flame from side to side; at the same time advancing and retiring 

 it gently all the while. By this method, you may proceed to 

 lay on smoke to any required darkness. It ought to be viewed 

 from time to time, not only to see whether it be sufficiently 

 dark, but whether any inequality may be perceived ; for, if that 

 should happen, it will not be proper to go on. 



The smoke of sealing-wax is bad : that of pitch is worse. A 

 wax candle gives a good smoke ; but that of a tallow candle is 

 better. As good as any I have hitherto met with, is the smoke 

 of spermaceti oil. In using a lamp, you may also have the 

 advantage of an even flame extended to any length. 



Telescopic Experiments. 



No. 1. By way of putting my theory to the trial, I used two 

 red glasses, and found that the heat which passed through them 

 could not be suffered a moment ; but I was now also convinced 

 that distinctness of vision is capitally injured, by the colouring 

 matter of these glasses. 



No. 2. I smoked a white glass, till it stopped light enough to 



