VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 685 



the orange; and a little of the yellow. It transmitted the green; stopped some of 

 the blue; but transmitted the indigo and violet. A blue glass intercepted much of 

 the red and orange; some of the yellow; hardly any of the green; none of the 

 blue, indigo, or violet. A purple glass transmitted some of the red; a very little 

 of the orange and yellow: it also transmitted a little of the green and blue; but 

 more of the indigo and violet. 



From these experiments we see, that dark green glasses are most efficacious for 

 intercepting red light, and will therefore answer one of the intended purposes; 

 but since, in viewing the sun, we have also its splendour to contend with, I pro- 

 ceeded to the following additional trials. White glass, lightly smoked, apparently 

 intercepted an equal share of all the colours; and when the smoke was laid on 

 thicker, it permitted none of them to pass. Hard pitch, melted between 2 white 

 glasses, intercepted much light; and, when put on sufficiently thick, transmitted 

 none. Many differently coloured fluids, that were also tried, I found were not 

 sufficiently pure to be used, when dense enough to stop light. Now, red glasses, 

 and the 2 last-mentioned resources of smoke, and pitch, any one of which, it has 

 been seen, will stop as much light as may be required, had still a remaining trial 

 to undergo, relating to distinctness; but this I was convinced could only be decided 

 by actual observations of the sun. 



As an easy way of smoking glasses uniformly is of some 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 ine- 

 quality 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. — N° 1. To put my theory to the trial, I used 2 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 in- 

 jured by the colouring matter of these glasses. N° 2. I smoked a white glass, till 

 it stopped light enough to permit the eye to bear the sun. This destroyed all dis- 

 tinctness; and also permitted some heat to come to the eye, by transmitting chiefly 

 red rays. N° 3. I applied 2 white glasses, with pitch between them, to the tele- 



