27 6> Dr. Herschel's Investigation of the Powers 



A darker green glass intercepted nearly all the red ; much 

 -of 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 proceeded 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 two 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 two 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 



