68 Dr. Brewster on new properties of heat, 
placed on a hot iron. The effects produced by the transverse 
combination will be understood from Fig. 7. (PI. II. ) in which 
C, C, &c. represent the similar fringes where the combined 
effect is produced, and DD, &c. the dissimilar portions where 
the difference of the effect is produced. The portions C, C, 
&c. will therefore polarise tints higher up the scale than any 
that are polarised by the plates singly, whereas the portion 
DD, &c. will polarise tints much lower on the scale than any 
of those polarised by the single plates. 
When the tints of the usual series are of the same intensity 
with those of the unusual series , the effect of crossing is very 
beautiful, and is represented in Fig. 8. (PL II.) where the 
portions corresponding to C, C, &c. are obviously affected with 
high tints in the scale, while those corresponding to DD, &c. 
are almost entirely black. The tint polarised in the central 
fringes of the plates, when separate, was the commencement 
of yellow. The combined tint, in the figure of a round cir- 
cular spot, was a beautiful indigo , which appeared at the centre 
of the square of intersection ABCD. This gradually shaded 
off through all the lower tints, and terminated in a dark cir- 
cular fringe. Beyond this fringe, towards the angles A,B,C,D, 
an opposite set of fringes were seen ; but towards the sides 
nothing but a dark shade was visible. All these phenomena 
are the necessary results of the principles already laid down. 
a.* 
Scholium. 
The phenomena described in the Proposition are the same 
as those which are exhibited by crossing plates of the two 
classes of doubly refracting crystals. The former, are how- 
ever, far more beautiful than the latter. 
