9 6 
Dr. Brewster on new properties of heat , 
The crystalline structure which exhibits the dark rectan- 
gular cross may be imitated, by forming a circle with various 
sectors of calcareous spar, having the principal sections of 
each directed to a common axis. 
Having had occasion to grind a part of a glass tube into the 
shape shown in Fig. 39. (PL IV.) I was surprised to observe, 
upon transmitting polarised light along its axis, and analysing 
it with calcareous spar, that it was depolarised in eight places, 
H 2, 3, 4, 5, 6, 7, 8, Fig. 40. (PI. IV.) When the line AS 
was parallel or perpendicular to the plane of primitive polari- 
sation, the tints were of the first order of Newton’s scale. 
The other image formed by the spar, had the appearance 
shown in Fig. 41. (PI. IV.) where the dark spots correspond 
to the white ones in Fig. 40. ( PL IV. ) 
. In order to discover the origin of these depolarising aper- 
tures, I cut another piece out of the same tube and polished 
the ends of the small cylinder, without grinding off any of the 
cylindrical circumference. When it w r as exposed to pola- 
rised light, it exhibited the appearance shown in Fig. 42. 
(PL IV.) where ACBD is a dark cross, separating four 
luminous sectors, and MNOP a dark circular space increasing 
in darkness towards the points M, N, O, P. If we now suppose 
the portions C a h, D c d to be cut off, something like eight 
luminous apertures will be left, as in Fig. 40. (Pl. IV.) This 
however is not the cause of the phenomenon. The four aper- 
tures on each side of the centre C, are the four diagonal 
fringes of the square pieces AC, BC, which act as if they were 
separated at C, the communication being nearly cut off. In 
this case, the cylindrical crystallization was converted into a 
rectangular crystallization by changing the shape of the 
glass. See Prop, XXV. 
