142 REPORT—1840. 
observed round the head on dewy grass * is evidently referrible 
to the reflexion from spherical drops at no great distance from 
one another. The peculiar luminous fringes observed round 
the whole shadow (308.), I have assured myself by many trials 
to arise from no optical deception. There is a space close to 
the shadow, and following its boundary, more luminous than 
the fully enlightened space beyond,—a fact which it seems 
not easy to explain. It appears to have no reference to the 
nature of the body (as, whether it is bedewed or not) which 
yields the shadow; nor is it altogether dependent on the body 
which acts as the recipient screen, but it depends chiefly, I ap- 
prehend, on the condition of the strata of air very near the 
ground. We must consider it, I suppose, as an effect of dif- 
fraction, such as would, in point of fact, be seen where the 
shadow of a body is thrown upon a screen by a single radiant 
point at a certain distance ; in this case, however, it is not the 
smallness of the luminous body, but the cluster of minute 
globules which reflect small images of the sun that are the cause 
of the diffraction ; the effect being to produce a luminous band, 
succeeded by a darker one whose contrast renders it more visi- 
ble. The effect is no doubt enhanced by the indeterminateness 
of the surface which reflects these little images. The shadow 
is seen through the whole depth of the nearly invisible cloud, 
and the bounding surface of light and shade will be most pro- 
minently seen when the eye is in a position to follow its course 
in the interior of the mist. 
311. The phenomenon described by Professor Necker}, of the 
intense illumination of shrubs and trees forming the horizon 
behind which the sun has just set, is, I conceive, a precisely 
parallel fact; and M. Babinet t has explained it in a similar 
manner. But there is still something in this as an optical phe- 
nomenon which seems to me to require further investigation. 
312. It should not be forgotten, with reference to the phzno- 
mena of clouds, that they have been treated by Young and others 
on the hypothesis of their being composed of spherical drops of 
water. Bouguer, Kamtz and Fraunhofer, maintain their vesi- 
cular structure ; and the last-named author has in his Memoir, 
so often cited, considered minutely the course of a ray of light 
through a spherical watery shell. The great modification which 
the common rainbow undergoes in a cloud, and the rarity of its 
* Garthe, dbhandlung tiber den Heiligenschein : quoted by Kiémtz, iii. 106. 
and other authorities there mentioned. 
t+ Philosophical Magazine, Third Series, i. 332, where two diagrams repre- 
sent the fact very well, as I remember to have seen it under Professor Necker’s 
directions. t Comptes Rendus, iv. 644. 
