Prof. Powell’s Postscript on the Dispersion of Light. 113 
II. In the Editors’ note appended to my paper (p. 28.) there 
is a reference to some investigations of M. Rudberg, pub- 
lished in a former volume of this Journal. I ought, perhaps, 
to have referred to those curious researches at an earlier pe- 
riod: but it will readily appear that they are quite distinct 
from mine. The author states at the commencement of his 
paper: “ In investigating the principle, according to which, for 
the explanation of the dispersion of light on the system of 
undulations, we must suppose that when the light passes from 
the air into a more refractive medium the length of the undu- 
lations are much contracted, in fact, much shorter,—I have 
found that the following relation appears to exist between the 
length of the undulation of a certain colour in the air and 
the corresponding one in any other substance :” L = a.1”; 
I being the length in air, Lin the medium, @ and m con- 
stant depending on the medium. 
He then takes Fraunhofer’s value of 7 for each ray, and 
assuming that they are diminished in proportion to the re- 
fractive power, proceeds to calculate L for the different media 
examined by Fraunhofer: and thence the refractive indices 
by the formula, which on this assumption follows from the 
former (N being the index) 
1 
N= ope 
and the resulting numbers certainly exhibit a very good agree- 
ment with those of observation. 
Now, with regard to the nature of the formula, it is to be 
observed that the author neither gives any theory from which 
it is deduced, nor a reference to any other paper, or investiga- 
tion of such theory; and the form of it is such as would ap- 
pear extremely unlikely to have any connexion with the ana- 
lysis of undulations. Again, had any such investigation either 
of the author or of any other philosopher been in existence, it 
is hardly conceivable that it could have remained since 1827, 
without becoming known to some, at least, of the numerous 
mathematicians in all parts of Europe who have since that 
period been directing their attention to the subject. 
But further, (unless I greatly mistake the author’s meaning, ) 
it appears to me from the very form of expression used in the 
passage above quoted, that the formula is not derived from any 
theory of undulations: for when, he says, “ In investigating, 
&c....... Lhave found, ......” the meaning really seems to me 
simply this; that in attempting such an investigation on the 
undulatory theory he had not been able to succeed in obtain- 
ing any theoretical relation between the yelocity of a waye, and 
