3j6 On the Powers of the prifmatic Colours 



To determine what glaflcs would nioft effe6liially (lop the 

 red rays, I procured fome of all colours, and tried them in 

 the following manner: — I placed a prifni in the upper part 

 of a window, and received its coloured fpe£lrum upon a 

 fheet of white paper. Then I intercepted the colours, juft 

 before they came to the paper, fuccefiively, by the glalfes, 

 and found the refult as follows : a deep red glafs intercepted 

 all the rays ; a paler red did the fame. 



From this we ought not to conclude, that red glafles will 

 flop the red rays ; but rather, that none of the fun's light, after 

 its difperfion by the prifm, remains intcnfe enough to pafs 

 through red glafles in fufficient quantity to be perceptible 

 when it conies to the paper. By looking through them di- 

 reftly at the fun, or even at day ob)e£ls, it is fuflficiently 

 evident that they tranfmit chiefly red rays. 



An orange glafs t ran fm it ted nearly all the red, the orange, 

 and the yellow. It intercepted fome of the green, much of the 

 blue, and very little of the indigo and violet. — A yellow glafs 

 intercepted hardly any light of any one of the colours. — A 

 dark green glafs intercepted nearly all the red, and partly alfo 

 the orange and yellow. It tranfmitted the green, intercepted 

 rnnch of the blue, but none of the indigo and violet. — A 

 darker green glafs intercepted nearly all the red, much of 

 the orange, and a little of the yellow. It tranfmitted the 

 grcCn, Hopped fome of the blue, but tranfmitted the indigo 

 and violet. — A blue glafs intercepted much of the red and 

 oranoe, fome of the yellow, hardly any of the green, none 

 of the blue, indigo, or violet. — A purple glafs tranfmittecj 

 fome of the red ; a very little of the orange and yi^llow. 

 It alfo tranfmitted a little of the green and blue, hut more 

 of the indigo and violet. 



From thefe experiments we fee that dark green glaffes are 

 moft efficacious for intercepting red light, and will therefore 

 anfwer one of the intendt d purpofes ; but fince in viewing 

 the fun we have alfo its fplcndcur to contend with, I pror 

 ceeded to the following additional trials : 



White glafs, lightly fmoked, apparently intercepted an 

 equal fliare.of all the colours; and when the fmoke was laid 

 gn thicker, it permitted none of them to pafs. Hard pitch, 



pelted 



