52 
Dr. Brewster on new properties of heat, 
glass, driving as it were before it a dark and undefined wave. 
Nearly at the same instant a similar but fainter white wave 
advances from the upper edge EF, driving before it a similar 
undefined dark wave ; and at no perceptible interval of time, 
another white fringe appears in a very diluted state about the 
centre a h, advancing towards the edges CD and EF. The 
waves of white light, which have their origin at the edges of 
the plate, and those which advance to meet them from the 
middle, have the effect of condensing the undefined dark waves 
into two black fringes MN, OP. A faint yellow wave next 
appears at CD, encroaching gradually upon the white one, 
and is followed by orange and red tints, completing the first 
order of colours in Newton’s scale. The colours of the second 
order next advance in succession, and the same thing happens 
with all the superior orders, so that three, four, and sometimes 
even nine or ten orders of colours are distinctly seen between 
MN and CD. When the green colour of the second order ap- 
pears at CD, a wave of yellow of the first order is seen at FE 
advancing upon the plate, and is followed by tints of orange, 
red, purple, &c. till several orders are distinctly visible be- 
tween FE and OP; and nearly at the same time another yellow 
wave developes itself at a h, and gradually encroaches on both 
sides upon the white fringe, but never reaches MN or OP. 
The yellow at a b next becomes orange, pink, purple, blue, green, 
&c. Each of these colours advances towards MN and OP, but 
never covers entirely the preceding colour, so that new fringes, 
sometimes to the number of six or eight, are thus formed 
between the black spaces. 
The terminal fringes are developed at the same time, and 
nearly in a similar manner. 
