AND ON THE COLOURS EXHIBITED BY CERTAIN FLAMES. 449 



emerald-green, and towards the back blood-red, passing in the 

 intermediate thicknesses through a kind of livid neutral tint. 



5. The blue glass above mentioned exhibits this change of 

 tint in a very marked manner. When a piece of the thickness 

 0.042 inch was employed, it separated the red portion of the 

 spectrum into two j the least refracted being a well-defined 

 band of perfectly homogeneous and purely red light, and se- 

 parated from the other red by a band of considerable breadth, 

 and totally black. This latter red was nearly homogeneous ; 

 its tint, however, differing in no respect from the former, and 

 being free from the slightest tinge of orange. Its place in the 

 spectrum is such, that its most refracted limit exactly comes 

 up to that remarkable black line which Dr Wollaston obser- 

 ved separating the red from the yellow in the solar spectrum. 

 A small sharp black line separated this red from the yellow, 

 which was a pretty-well defined band of great brilliancy and 

 purity of colour, of a breadth exceeding that of the first red, 

 and bounded on the green side by a dark, but not quite black 

 interval. The green itself was dull, and ill defined ; but the 

 blue and violet seemed to be transmitted with very little loss. 

 The type of this glass is as in Fig. 2. 



A double thickness (0.084 in.) obliterated the second red, 

 but left the first unimpaired. The yellow was greatly en- 

 feebled, and somewhat reduced in breadth, and terminated on 

 the side of the green by a black band, equal in breadth to it- 

 self. The green also was extremely diminished, but the blue 

 less so, and the violet remained nearly as before. The curve, 

 Fig. 3., exhibits the type of this thickness of the glass. Last- 

 ly, When a great many thicknesses of it were laid together, 

 the extreme red and the violet only were transmitted, and the 

 type assumed the character of Fig, 4. 



S l 2 The 



