444 EXPLANATION OF PLATE XXVII. 



Blue, and, last of all, the Indigo ; so that the combinations of Blue 

 glasses have now the effect of Red glasses. 



No. 5. Shews the effect of increasing the Blue glasses in No. 4. to a certain 

 thickness. 



No. 6. Shews the effect of a Sky-blue paste, which reflects most copiously the 

 Blue light. The whole spectrum is destroyed excepting the extreme Red. 



No. 7. Represents the effect produced by a great thickness of Green glass. 



No. 8. Sulphate of copper, which is bluish-green, both in the solid state and in 

 the state of a solution, leaves unabsorbed a great quantity of Yellow 

 light, which may be rendered tolerably homogeneous by means of a 

 pale Yellow glass. The figure shews its effect in attacking the Red and 

 Violet ends of the spectrum, a portion of blue and red being left. 



No. 9. Shews the effect of a greater thickness of Sulphate of copper, in absorbing 

 all the Red, and leaving a Greenisl^-yellow, with an adhering margin of 

 blue light. 



No. 10. Shews the influence of a thick plate of Yellowisli-red glass, in absorbing 

 all the blue, and part of the green, leaving the violet slightly affected. 



No. 11. Represents the singular effect produced by a solution of Lake. Tlieiv 

 are here two greens, and an effect is produced at the boundary of the 

 red and green space. 



No. 12. Represents the effect of a thick piece of fine Red glass, coloured with gold, 

 and also of a piece of stained glass, which I found in the Abbey of 

 Konigsfelden, in the canton of Berne. The same effect is produced by 

 common red ink, and by a solution of beet-root in vinegar. 



No. 13. Shews the action of an opaline milky- white glass upon the most refran- 

 gible half of the spectrum. 



No. 14. Shews the effect of a certain thickness of native yellow orpiment. 



No. 15. Shews the effect of a red glass combined with yellowish-green and bluish- 

 green glasses. The yellow is much more copious and brilliant when 

 the red glass is combined with a certain thickness of sulphate of cop- 

 per, but it is then fringed with a narrow margin of Red on one side, 

 and of Green on the other. 

 No. 1 6. By combining with the same red glass a thicker plate of sulphate of cop- 

 per, the yellow becomes green ; and by a. thicker plate still, the colour 

 becomes blue, as in the figure. 



XXXL 



