IN PLATES, TUBES, AND CYLINDERS OF GLASS. 367 
negative structure from the two external positive structures. 
The tint at a, or any part of the line bc, would be 
Te 
—— 32 BE but if the circular plate were part of a plate si- 
milar to, and at right angles to ABCD, the tint at a, or any 
part of the line EOF, would be equal to T; and as this tint is 
‘rectangular to the other tint at a, the resulting tint will be 
equal to the difference of these tints, or to 
ete Td ae 
| ee ada Beale He 
In like manner, it may be shewn, that in every point of the 
2 
circular plate, the tint is represented by ae which is the 
experimental expression for it already found. In plates, 
therefore, that have only a positive structure, the negative 
structure still exists, but is overpowered by opposite ac- 
tions. 
We are now prepared to understand how the negative struc- 
ture re-appears, as shewn in Fig. 11., by giving an elliptical form 
- to the plate. For, the maximum negative tint produced at O, 
in the direction gh, is no longer counterbalanced by the tint 
in the direction ef; and therefore the difference of these tints 
appears at O, with a negative character. As the points e, f re- 
move from O, oras the ellipticity increases, the tint at O gradual- 
ly rises till it becomes equal to T, or a times the tint at 
g, when the action of the edges at e and f has no longer any 
influence at O. The very same results are obtained by the 
conversion of a sphere into a spheroid, and they are explicable 
upon the same principles. 
The 
