112 Dr. Brewster on new properties of heat , 
method of using the wedge or nonius is shown in Fig. 55, (PL V.) 
"where AB is the wedge, exhibiting tints which vary in in- 
tensity from A to B. If we wish to ascertain the tints of a 
piece of crystallized glass CD, it must be held as in the figure, 
and moved from A to B. When it has the position CD, the 
intersectional figure is open horizontally, which shows that 
the tints of AB, at the point m, are higher than those of CD. 
In the position GH the figure is open vertically, and therefore 
the tints of the wedge at 0 are lower than those of the plate. 
But in the intermediate position EF, a dark cross is produced, 
which evinces the perfect equality between the tints of the 
wedge at n and those of the plate EF. In this manner all tints 
may be compared with each other, and referred to the scale of 
colours. 
By forming wedges of crystallized glass in this way, we 
are enabled to observe the gradations by which the tints pass 
into each other, and to perform many experiments on the 
orders of colours, which would otherwise have been imprac- 
ticable. 
The sensibility of the preceding instrument depends on 
several other causes. 1st. On the intensity of the polarised 
pencil. 2d. On the transparency of the glass. And 3d. On 
the removal of all internal reflections at the junction of the 
plates. In the instrument with 20 plates already mentioned, 
the glass has a green tinge, and the polarised light suffers no 
fewer than 40 reflections before it reaches the eye. In order 
to remove these evils, the light should be polarised by re- 
flection from several of the thinnest and most colourless plates 
of glass that can be procured, so that each plate may polarise 
and reflect the light which is transmitted through the plate 
