370 ON THE DISTRIBUTION OF THE POLARISING FORCE 



The thickness of C D was .230, and its tint 10.2. But 

 133 2 :230 2 =: 3.4: 10.2; so that the tints and the elasticities 

 were, in this case, inversely as the squares of the thicknesses. 



The preceding experiment furnishes us with the principle of 

 a new instrument, which may be called a Teinometer, for ascer- 

 taining the elasticities of bodies. The tints of AB and CD are 

 obviously measures of the elasticities of the two plates ; so that 

 the elasticities of plates of glass of different dimensions, or form- 

 ed of different materials, may be readily determined. The power 

 of the instrument however, is not limited to glass ; for, by 

 usino- a glass plate AB as a standard, the other plate CD may 

 be a plate of metal, or any other opaque substance, whose elas- 

 ticity it is required to ascertain. The tint of AB, when op- 

 posed by a plate of equal elasticity, is known by experiment ; 

 and therefore, the tint which it affords, when opposed by a si- 

 milar plate, of a substance possessing a greater or a less elasti- 

 city than itself, is a measure of this elasticity. Although I 

 consider the variation of the tint as an excellent means of de- 

 termining the degree of curvature, yet the principle o exhibiting 

 the relative elasticities of two plates, by the application of the 

 same force, may be employed in an instrument entirely me- 

 chanical, in which the sagitta of the inflected plate is actually 

 measured, as in the ingenious machine invented by S'Graves- 



ande. 



The chromatic Teinometer is represented in Figs. 14, 15. 



and 16. where AB is the standard plate, of well annealed 

 glass, having its edges highly polished. Along this plate, 

 there are moved two brass pieces, S a b c, S' a b' c', which can 

 be fixed in any position, by means of the screws S, S . The 

 plate CD, where elasticity is to be measured, rests with its lower 

 edge upon the projection be, Fig. 15. and with one of its faces 

 against ab. It is then pressed into contact with the plate AB, 



and 



