54 Dr. Brewster on new properties of heat , 
blue fringe was converted into a yellowish green , a tint due to 
the sum of their actions. Hence it follows, that the blue 
in the second fringe below MN is a blue of the second order. 
Similar results were obtained by combining the sulphate of 
lime with the parts of the glass which produced the other sets 
of fringes. 
Another proof of the proposition was obtained in the fol- 
lowing manner. I took two plates of thick glass, and having 
placed them on a hot iron, as before, I waited till all the fringes 
had disappeared except the white of the first order. When 
one of the plates was lifted vertically, so that the portion of 
the glass CDNM was opposite to a b , the two white fringes 
produced a black tint. When the same plate was depressed 
till a b of the one plate was opposite to CD of the other, the 
white fringe above CD was also converted into black. This 
black, however, w r as not so deep as before, as the white in the 
exterior fringe is brighter than in the interior one. In the first 
case, this superiority was compensated by the cooling of the 
glass at CD, in consequence of its being lifted from the hot 
iron, whereas in the second case, the cooling had not affected 
the interior part a b. When, on the contrary, the one plate was 
held in such a position that its fringes were at right angles to 
those of the other, as shown in Fig. 3, (PI. II.) the white of 
the exterior fringes of the one plate combined with the white of 
the exterior fringes of the other, produced black. Ihe white 
of the interior fringes of the one plate, when combined witli 
those of the other plate, produced black, and the white of 
the interior fringes of the one plate, when combined with the 
white of the exterior fringes of the other, produced a brighter 
white. 
