&o Dr. Brewster on new properties of heat , 
By comparing the numbers in the 5th column, which are mil- 
lionth parts of an inch, with those in the second column, it 
will be found that the constant factor, by which we must mul- 
tiply the thickness of any plate of glass, in order to obtain 
the thickness of the plate which would afford by reflection a 
tint similar to its maximum tint, is nearly tttso* 
It is a curious circumstance, that the permanent fringes 
have precisely the same character as the transient fringes 
which are produced by placing glass plates upon a hot iron, 
while the transient fringes, developed during the cooling of 
glass plates, have an opposite character. 
The limiting temperature at which the former are changed 
into the latter, is probably that, at which the permanent struc- 
ture is communicated. 
When the glass plates are cooled more at one edge than at 
another, the fringes are less distinct, and the tints lower at 
the edge that is least rapidly cooled. This difference becomes 
more perceptible as the height of the plates is increased. 
When the plates of glass are thick, and exposed to a con- 
siderable heat, they often lose their polish, and exhibit on their 
surface a delicate fibrous texture when examined by a micro- 
scope. This texture sometimes consists of grooves which 
exhibit by reflection the coloured images produced by mother 
'Of pearl. It also communicates the same property to wax. 
