of Chromatic Dispersion. 177 
Oil of Cassia. Diff. Oil of Anise. Diff. 
No. 1. «=1:477811 . . 2621 No. l. e=1:478606 . . 1217 
2. 1-475190 . . 5569 2. 1:477389 . . 3813 
3. 1-469621 . . 8190 oe 1:473576 . . 50380 
The above correction may be effected by an alteration of the 
indices so small as to be of no account. 
This law, then, that the indices of elasticity of the ether in 
the pores of any medium are in the inverse order of the tempe- 
ratures, and the differences of the indices are proportional to the 
differences of temperature, may be regarded as rendered highly 
probable by these two cases, being the only media on which we 
have observations at more than two temperatures, so as to illus- 
trate this point. 
The subsistence of this law in the case of oil of anise is all the 
more remarkable, because the indices of ail the fixed lines are there 
greater in No. 2 (the intermediate temperature) than in either 
No. 1 or No. 3, thus showing the absolute magnitudes of these 
indices alone to be an imperfect criterion by which to judge of 
the condition of the ether within the pores of the medium. This 
law thus removes an anomaly which would otherwise be presented 
by the oil of anise, in which, were we to judge by the indices of 
the fixed lines alone, we should be led to infer that, in passing 
from temperature 13°°25 to temp. 15°1, the enlargement of the 
pores is attended by an increase in the tension of the «ther,—a 
result in the highest degree improbable. The foregoing investi- 
gation shows that this is not the case, but that the tension of 
the zether, as determined by the value of ¢, does actually dimi- 
nish with the enlargement of the pores, consequent on the rise 
of temperature. 
This law is important in reference to the undulatory theory, 
being exactly what it would lead us to expect; and it confirms 
the conclusion that the quantity ¢ is the true index of the elas- 
ticity of the «ther within the pores of the medium. 
On comparing together the two media—the oils of cassia and 
anise—it will be perceived that in the former the rise of 12°°5 of 
temperature, from temp. 10° to temp. 22°'5, gives on the value 
of € a decrease of 0:008190; and in oil of anise, the rise of 7°65 
from temp. 13°°25 to 20°9 gives on € a decrease of 0:005030. 
These two are so nearly proportional to each other, as to lead 
to the inference that in two different media, in which the elas- 
ticity of the wether is nearly the same, the effects produced by 
a given change of temperature are also nearly the same. 
From the foregoing, it appears that the effects of temperature 
in altering the action of any medium on the light passing through 
it are so considerable, as to render it highly desirable that obser- 
vations should be made on each medium at six or seven different 
