6o 
Dr. Brewster on new properties of heat , 
These two parcels of plates were then placed upon the hot 
iron, and the one parcel exhibited the same tints as the other, 
both in the exterior and interior fringes. 
In order to prove the second part of the proposition, I took 
three parcels, one of two plates, another of four plates, and a 
third of six plates, all of them having been cut out of the same 
mirror. I then placed the different parcels upon the hot iron, 
and when the colours were perfectly developed, I held the 
system of four plates in a position transverse to the system of 
six plates as shown in Fig. 3. (Pl. II. ) A broad fringe of blue 
light of the second order appeared at the intersection of the 
central lines a b, a'b'. The very same colour was polarised 
by the system of two plates, whose united thickness was equal 
to the difference of the thickness of the transverse parcels. 
See Prop. XV. 
Proposition XI. 
The number and form of the plates of glass remaining the same , 
the tints which are polarised at the central line a b, and at the 
edges CD, FE, Fig . 2, (PL II. ) ascend in Newton’s scale as 
the temperature of the source of heat is increased. 
I took a thick plate of mirror glass 6,9 inches long, 2,27 
inches high, and 0,163 thick, and having placed it upon a 
heated iron, which just appeared red hot in the dark, I found 
that it polarised the green of the second order in x\\q first ex- 
terior set of fringes, and the greater part of the white 01 the 
first order in the second exterior set of fringes. When the 
heat was more intense, the same plate polarised th egteen of 
the third order in the first exterior set of fringes. 
When 15 plates of mirror glass were placed upon the top 
A- 
