of Chromatic Dispersion. 265 
Powell are so very inferior to those of Fraunhofer, it should not 
occasion surprise to find that in some instances this inferiority 1s 
still more marked, and that some of Powell’s observations, when 
tested by the exponential law, should be classed as only of the 
tenth order. Of this we have an example in the case of nitrate 
of potash, which is of low dispersive power, and should therefore 
have presented no peculiar difficulties. But this case belongs to 
a class of observations of which the observer himself says, that 
the media being of low dispersive power, and considered by him 
of little importance, the calculations were in consequence carried 
to only a slight degree of approximation. So far, then, from 
the discrepancies, in the case of nitrate of potash, tending to 
shake confidence in the exponential law of the indices, they 
ought to be regarded as strengthening its probability, by show- 
ing it to be capable of detecting the errors in these observations 
which might otherwise have escaped notice, seeing that, in con- 
formity with the observed indices, nitrate of potash belongs to 
the class of regular media; and there is no other test by which 
the errors could have been brought to light. 
This example shows that, with reference to any general law of 
dispersion, no medium ought to be viewed as of small import- 
ance, but that the same attention should be given to secure 
accuracy in media of low, as in those of high dispersive power. 
Of the twenty-nine observations by Powell which have been 
tested by the exponential law of the indices, there is not one 
that can be classed as of the first order—a fact sufficiently indi- 
cative of their general inferiority. There are, however, thirteen, 
or nearly one-half, which are of the second order, and may there- 
fore be regarded as fair observations. These are— 
Sulphate of magnesia. Nitrate of bismuth. Superacetate of lead. 
Solution of potash. Nitrate of lead. Nitric acid. 
Sulphate of soda. Subacetate of lead. Oil of sassafras. 
Alcohol. Muriate of ammonia. Oil of anise, T. 13°°25. 
and the same at temp. 20°-9. There are five of the third order, 
muriatic acid, nitrate of mercury, muriate of lime, rock-salt, and 
oil of anise, temp. 15°°8. There are two of the fourth order— 
sulphuric acid and creosote ; three of the fifth order—pyrolig- 
neous acid, bisulphuret of carbon, and muriate of baryta; one of 
the sixth order—oil of cassia, temp. 14°; two of the seventh 
order—solution of soda, and oil of cassia, temp. 10°; one of 
the eighth order—water, temp. 15°°8; one of the ninth order 
—oil of cassia, temp. 22°°5; and one of the tenth order—solu- 
tion of nitrate of potash. From this enumeration it is clear 
that there is no connexion between the amount of error and the 
dispersive power of the medium, seeing we have media of low 
