as exhibited in its propagation along plates of glass. 59 
number of plates was increased, and were always such as 
belonged to a thickness taken in proportion to the number in 
the third column of Newton's scale. 
When one plate, for example, polarised at ab, a yellow of 
the first order, two plates gave an indigo of the second order, 
three a red of the second order, four a green of the third 
order, five a bluish red of the third order, and six a yellowish 
green of the fourth order. Now the numbers representing 
these tints in Newtons scale, are nearly 4, 8, is, 16, so, 24, 
and the corresponding thicknesses are 1, 2, 3, 4, 5, 6. A 
variety of other experiments were made with the same result. 
Proposition X. 
If a number of glass plates of the same form and of the same chemi- 
cal composition, but of various thicknesses, are placed upon a hot 
iron , then if two or more of them are combined symmetrically , 
that is with their edges CD coincident , the colour polarised in any 
part will be the same as that which would have been polarised 
by a single plate having a thickness equal to the sum of the 
thicknesses of the plates ; but if the plates are placed transversely , 
or with their edges CD at right angles to each other, the colour 
polarised at those parts of the glass, which are similarly situated 
with regard to the black spaces , is the same as that which would 
have been polarised by a single plate, whose thickness is equal 
to the difference of the thicknesses of the two transverse plates 
or systems of plates • 
I took two plates of mirror glass that had different thick- 
nesses, but nearly the same colour, and having cut them into 
equal rectangular pieces, I foUnd that three of the one had the 
same thickness as five of the other. 
I 2 
