16 SEVENTH REPORT—1837. 
character. In these cases, instead of a brilliant ray rising 30° or 40°, 
the luminous appearance was rather that which would present itself, if 
a series of images of the sun were superposed, in the line of its vertical 
diameter, and extending over not more than 4° or 5°; the edges were 
ill-defined. In his letter to Professor Forbes, Mr. Christie had sug- 
gested, whether the phenomenon could be due to a series of reflec- 
tions of the sun’s image by strata of thin cloud ; but he now suggested, 
whether such a phenomenon would not be presented by successive 
reflections on the undulating surface of a stratum of liquid air, such as 
M. Poisson, in his new Theory of the constitution of the Atmosphere, 
has supposed to exist. 
On Von Wrede’s Explanation of the Absorption* of Light, by the undu- 
latory Theory. By Proressor Powe... 
Von Wrede supposes the particles of a transparent medium to be 
placed regularly, at equal distances, () so that the ather being dif- 
fused among them, the series of waves constituting a ray of light, can 
be propagated directly through the substance ; yet a portion of each 
wave will encounter some of the particles, and be reflected backwards, 
and then forwards again, and at length emerge along with the di- 
rectly transmitted ray, and interfere with it, the conditions of which 
will depend on the amount of retardation, or differences of the 
phases; which, if amounting to odd multiples of the half wave- 
length (A), will give points of darkness; and if to even multiples, 
points of brightness. These may be confounded in compound light, 
but will appear when the rays are separated by the prism, and give dark 
bands in the spectrum. 
He then investigates a formula for the intensity of a system of 
waves compounded under the conditions supposed. This is deduced 
from the ordinary formula for the velocity of the wave, and is ulti- 
mately brought into a form including certain terms dependent on the 
medium, and constant for the same medium, together with the factor 
2b 
cos 27 eC 
which is so involved that the intensity is a maximum when the cosine 
26. : cee. 
becomes = + 1, or when 2 = is ameven multiple of a semi-cireum- 
ference, and a minimum when the cosine =—1, or when 27 2, an 
odd multiple of a semi-circumference. 
Hence if the medium be such that 2d = * for any primary ray, that 
ray will be at a minimum, or will appear absorbed. If 26 be less than 
* See “ An Attempt to explain the Absorption of Light according to the Undulatory 
Theory; by Baron Fabian yon Wrede,” in Taylor’s Foreign Scientific Memoirs, vol. i. 
p. 477. 
