of Chromatic Dispersion. 171 
corresponds to d,; whence we have 
(“C—"C) + (“D—"D): (C—"C) ::*D—"C : "C+, 
whence vy, may be found; so also 
(“P—“F) + “G—“G) :“F—“F::"G—"F :"F+y,, 
whence v, may be found. ‘Thence should arise respectively 
*N, =v, and “N,=v,; that is, the refraction of the node should 
be the same as the corresponding node of the refractions. 
When the corrections on the refractive indices, and the corre- 
sponding extrusions required by these laws, shall have been 
made where necessary, there will be obtained a series of values 
of these two sets of quantities perfectly self-harmonious, all the 
errors arising from inaccuracy of observation having been made 
to neutralize each other. The resulting figures will be found to 
agree so nearly with those obtained from observation, as to leave 
not a doubt of the real existence of the several laws which have 
been thus ascertained, and of the accuracy of the method of in- 
vestigation pursued. 
So much for the regular media, which all present the foregoing 
characteristics ; and now for the peculiar, The only media yet 
ascertained to fall under this class are the following eight :— 
Alcohol. Pyroligneous acid. 
Oil of cassia. Muriatic acid. 
Oil of anise, T. 15°. Solution of soda. 
Muriate of zine. Sulphuric acid. 
The peculiarities presented by these media, as respects their 
extrusive property, are of three kinds :—l1st, an alteration in the 
position of the nodes; 2nd, an alteration in the character of the 
transfer of motive energy ; and 3rd, an alteration in the number 
of the nodes, involving, as a consequence, both the other two 
peculiarities. 
Alcohol has its upper node between B and C, its lower between 
F and G; it may therefore be termed high nodal. Oil of cassia 
has its upper node in the usual place between C and D, but its 
lower between G and H; it may therefore be termed low nodal. 
Oil of anise, at temp. 15°, is also low nodal; but at the other two 
temperatures examined it is regular. The muriate of zinc pre- 
sents two peculiarities. It is, like aleohol, high nodal; but it is 
also medio-negative; that is, the transference of motive energy is 
from the central to the terminal parts of the spectrum, bemg 
the reverse of what is observed in regular media. Its nodes are 
between B and C and E and F, near F. 
The remaining four present more than two nodes, thus invol- 
ving an oscillation of the transfer of motive energy. Pyrolig- 
neous acid has three nodes—between Band C, C and D, Gand 
