as exhibited in its propagation along plates of glass. log 
the tints, corresponding to several points in the thermometri- 
cal scale, have been accurately ascertained. 
An instrument of this kind which I have constructed, is-re pre- 
sented in Fig. 51 , (PI. V. ) where ABC is a series of 20 plates of 
glass, whose length AB is 3.2 inches, their breadth 1.2 inches, 
and their united thicknesses BC 5.4 inches. A metallic vessel, 
DEFG, has its bottom formed of a thin layer of tin or lead, 
or any other suitable metal which can be poured in a fluid 
state upon the upper edges of the glass plates, so as to touch 
them in every part. This perfect contact may be obtained for 
higher temperatures, by grinding the bottom of the metallic 
vessel till it touches the edges of the glass in every point. 
When a heated fluid is poured into the vessel DEFG, its 
heat will be instantly communicated to the edges of the plates, 
and when exposed to a polarised ray, subsequently analysed 
by reflection from a transparent body, they will exhibit the 
coloured fringes at AB. Now every tint in the scale of 
colours has a corresponding numerical value, which becomes 
a correct measure of the temperature of the fluid. 
Instead of pouring the fluid into the vessel, we may remove 
the vessel altogether, and plunge the glass plates into the 
fluid. They must then be taken quickly out and suspended 
in a position where they are properly exposed to polarised 
light. The maximum tint which they develope at the centre, 
while cooling, is a measure of the temperature which they 
have acquired in the fluid. 
In order to obtain some idea of the nature of the scale, I 
made the following trials.- — The heat of my hand when 
applied to the edges of 20 plates of glass, produced instantly 
the fringes with the black spaces. With 12 plates I have 
