484: Queries and Answers. 
interesting to some of your readers. It was picked up in a quarry on 
Whitcliff Hill, adjoining the fine demesne of E. L. Charlton, Esq., near 
Ludlow, Salop, some time last summer, and came into my possession in the 
beginning of this year, through the hands of the person who found it. It 
appears to me to be a fine specimen, and very perfect, with the exception 
of part of the head. It is embedded in micaceous sandstone ; but the hill 
in which it was found is, I believe, chiefly composed of limestone. I here- 
with send you a sketch of it; but, being a rough draughtsman, it is very 
much inferior to the original. I should be glad to be informed, in your 
next Number, of the systematic name of this trilobite, as it appears to me 
that none of the species figured in your Magazine are of this kind. I am, 
Sir, &c.—John Evans. Broad Street, Worcester, March 12. 1830. 
We sent the original to Mr. Sowerby, who is not able to determine the 
species. — Cond. 
The Eagle-stone, or Etttes. — This is a hollow, crustated, or flinty stone, 
containing a nucleus within, which rattles when shaken. Large strata or 
beds of the coarse sort are said to be found near Trevoux in France, within 
a few feet of the surface. The popular tradition of it is, that it is carried 
to the eagle’s nest while sitting, in order to prevent her eggs from rotting ; 
and Mathiolus goes so far as to say (Rees’s Cyc., vol.i, A¢tites) that birds 
of prey will go in search of it as far as the East Indies. The form is usually 
a rounded knob, rather approaching to the kidney form ; the surface is foul 
with earth, but the external rind is metallic, and nearly as glossy as silk. 
The kernel has not any lustre at all. Can you or any of your readers in- 
form me where it may be found in this country ? — H, D. 
Effects of Heat and Cold, relative to Vision; in answer to Y. (p. 200.) — 
The apparently diminished size and distance of the objects in both the cases 
noticed by Y., under “ Effects of Heat and Cold relative to Vision,” at 
p. 200., admit of an easy explanation, without reference to the influence of 
either heat or cold. The intensity of light must necessarily decrease, not 
only as the distance of the centre from which it radiates increases, but in 
proportion also to the imperfect transparency of the medium through which 
it passes. The eye accustomed to these facts, under common circumstances, 
judges with tolerable accuracy of the comparative distances of objects ; but 
a greater or less degree of transparency in the atmosphere will be found 
considerably to disturb this judgment : a fog, for instance, is said to magnify 
objects, when the truth is, that, by diminishing the intensity of light, it 
makes objects seen through it appear farther distant, without lessening the 
visual angles subtended by them; and because an object at two miles, sub- 
tending the same angle as an object at one mile, must be twice as large, the 
conclusion is drawn that the dim object is large. The reverse takes place 
when the atmosphere is unusually pure and transparent ; for then the lights 
of objects appearing bright and vivid, while the visual angles they subtend 
are not altered, they appear diminished in size and distance. But the eye 
judges of the distance and size of an object, not only by the brightness of its 
light, and by the magnitude of the angle it subtends, but in a great measure, 
also, by comparing it with other objects intermediate and lateral. Now, in 
both the instances quoted by Y., these material aids were wanting: in the 
first, from the absence of other objects; and, in the second, from the situ- 
ation of the spectator excluding them from view. 
The kind of Mirage noticed by your correspondent Y., I presume, is that 
sO common in intertropical plains in hot dry weather, when the traveller, 
looking round, fancies he sees on all sides extensive lakes, reflecting the 
images of the distant mountains, of the islands with which their surfaces 
appear studded, and of the trees and buildings which adorn their margins, 
or rise up in the midst of their waters. This phenomenon, when seen for 
the first time, seldom fails to deceive a stranger, who, it may be supposed, 
is pleased enough to haye such a prospect where he expected “ a dry and 
