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EXPLANATION OF PLATE XXVII. 



Fig. 1. Represents one form of the Monochromatic Lamp, where A is the reser- 

 voir containing the diluted alcohol, which descends by the channel ABCD 

 to the broad wick E, whieh is generally made of sponge. A frame of 

 wire-gauze F moves round a hinge H, so that it can be brought over 

 the flame, and made to descend, when hot, upon the surface of the wick. 

 Excellent wicks may be made with concentric cylinders of thin mica, or 

 of platinum foil. 

 Fig. 2. Is another form of the Lamp, without a wide, in which the diluted al- 

 cohol is burned in a flat platinum or metallic dish MN, which may be 

 made to have a slight spontaneous oscillatory motion, for the purpose of 

 bringing the fluid over the heated projections of the platinum. A com- 

 mon spirit lamp OP, inclosed in a case, is placed below the platinum 

 dish MN, in order to produce sufficient heat for throwing off the vapour 

 from the diluted alcohol. 



A chimney, or a cylinder, of pale yellow glass may be placed round the 

 flame, if it should be thought of any consequence to absorb the small por- 

 tion of blue light which accompanies the yellow flame. 

 Fig. 3. Represents the method of finding the position of the Yellow space in the 



spectrum. 

 Fig. 4. Represents the various ways in which the prismatic spectrum is attacked 

 by the absorbent action of differently coloured glasses, as described in the 

 paper. 

 No. 1. Represents the prismatic spectrum as described by Dr Wollaston. 

 No. 2. Represents the effect produced by viewing the spectrum through a blue 

 glass, like that used by Dr Young. The spectrum is attacked at 

 a, b, c; a portion of the middle of the Red space being destroyed at a, 

 a portion of the Red and Green, disclosing the Yellow at b, and a por- 

 tion of the Violet being destroyed at c. 

 No. 3. Shews the effect of a different kind of blue glass, which destroys the in- 

 ner portion of the Red space at a, a less portion of the Violet at c, and a 

 greater portion of the Green at b ; the disclosed Yellow being now of a 

 different hue. 

 No. 4. Shews the effect produced by the Blue glasses of No. % and No. 3. com- 

 bined. The spectrum is now attacked at a fourth point, viz. the com- 

 mencement of the Blue space at c, and the Yellow is rendered less bril- 

 liant. By increasing the thickness of these plates of blue glass, the 

 dingy Yellow part vanishes, then the Green, as in No. 5., then the 



Blue, . 



