1825.) 
ticed ; but when these concur to favour it, 
the illusion is. irresistible, atid its’ cause 
strikingly’ difficult’ of. detection. | The’ de- 
gree of curvature varies as’ the’ situation of 
the spoke witly respert'to the perpendi- 
cular: |The} two ‘spokes, which arrive at 
the vertices; above and below the axle, are 
seen without curvature. The others, as 
more’ ori Jess. remote from. these, vary in 
ee of curve’; which is always outwards, 
whatever “be the direction, in which the 
wheel! may be'moving. © In. order to 
explain this phenomenon, it is necessary to 
observe the ‘influence of certain variations 
of circumstances upon it.—1. Velocity (in 
the motion of the wheel) is necessary to 
produce the deception in question. If this 
velocity be gradually communicated, the 
appearance is first pereeptible in the more 
horizontal spokes: this being observed, a 
small inerease of velocity suddenly produces 
the same. appearance in all the lateral 
spokes. -.The velocity, if not. so great as 
to prevent the eye from distinctly following 
the lines (ferit may, evidently, be increased 
to sucha degree, as to render them invisi- 
ble), has little effect on this curvature ; but, 
however rapid-the movement may be, each 
spoke will appear, for the instant, to be at 
rest—2. The number of spokes in the 
wheel makes no difference in the degree of 
curvature exhibited—3. The appearance is 
more perfect when the bars are narrow, 
provided they are sufficiently wide to allow 
a distinct, view of all the parts of the wheel 
in succession, and when the colour of. the 
bars is dark, and a strong light is thrown 
on the wheel- The deception is also aided 
by any occurrence, tending to fix the at- 
tention on the wheel—4.. If the number of 
bars be increased, no other difference than 
a greater multiplication of curved spokes 
will appear; but a certain relation being 
preserved, between the angles subtended by 
the eye, bars and spokes, will correct this. 
The distance between the bars and the 
wheel is of no consequence, provided the 
latter be not very near the eye, as, in that 
case, the aperture may. allow too large a 
ion of the wheel to. be seen at once— 
5. No curvature appears when the spokes 
are parallel.to bars, which, not being ver- 
tical, are somewhat inclined towards the 
horizon, but, in that-case, the relations of 
the other,spokes are the same. When the 
inclination.of. the bars is considerable, the 
images. become much. more crowded, and 
the distinctness of the appearance dimin- 
ished. _ This.deception totally ceases when 
the bars are parallel to the line of motion 
—6. This eff 
when progressive and rotatory movements 
take 
Ml, without revolution, it will not 
“hac. Aeln, hsid'« progr 
be given to" the ‘bars, ‘while’ the 
wheel revolves on a fixed axis; or if the 
Monrury Mac. No. 414. 
Spirit of Philosophical Discovery. 
145 
wheel (thus revolving) be viewed through 
fixed bars, by a spectator moving to» the 
right ‘or left; the curved appearance will 
immediately be assumed ;. such motion pro+ 
ducing an alteration, in the field of visions 
in the relative situation of the bars andthe 
wheel. noc 
The above facts justify the conclusion, 
that this deception must arise from the 
circumstance that parts only, of the:spokes, 
are seen .at any given. moment,—the.re- 
mainders being then covered by the bars. 
Yet why, since several parts’ of the same 
spoke are seen in the same straight line, 
does not imagination lend its powerful aid, 
as in other cases, to complete the impres- 
sion? _ The first idea is. that the portions, 
thus seen separately, are connected with 
portions of adjoining spokes, and the cur- 
vature thus apparently produced. But a 
little attention to the phenomena will 
show that such solution cannot apply : for, 
if the dise of the wheel, rolled behind the 
bars, have only one radius marked, instead 
of a number of radiant lines, it presents. the 
appearance of a number of radii, determined 
by that of the intervening bars. So that, 
evidently, several portions of one and the 
same straight line (seen through the inter- 
vals of the bars) form, on the retina of the 
eye, the images of so many distinct radii. 
The true principle, then, on which this 
phenomenon depends, is the same as that 
to which is referred the appearance of a 
line of light, occasioned by a bright object 
wheeled rapidly round in a circle; viz. 
that an impression on the retina, made 
by a pencil of rays, if sufficiently vivid, will 
remain, for a certain time, after the cause 
has ceased ; and many analogous facts,.as 
to the other senses, have been observed, 
which fully support this conclusion. 
- The Extraordinary Durability of Human 
Hair has been shewn, by some late experi- 
ments undertaken by Professor Pictet’ of 
Geneva: he compared some hair obtained 
from a mummy brought from Teneriffe, 
with some recent hair, in the construction 
of some hygrometers ; and, to his surprise, 
found the hair from the muxamy equally 
sensible to the smallest changes in humi- 
dity, with the usual specimens of human 
hair recently cut off. 
The Overland Expedition to the Arctic 
Regions, under the command.of..Captain 
Franklin, has hitherto made a -successful 
progress, according to a recent letter from 
Dr. Richardson to. Professor Jameson, 
dated from the Canadian or western extre- 
mity of Lake Huron. ‘The whole party 
were in good health, and expected to reach 
their winter-quarters about the end of 
September. Dr. Richardson states, t 
nothing new, in the form of scientific in 
mation, Had. occurred the i 
ing been employed in.m 
$ropeel } soe bao 
of the winter season. 
PROCEEDINGS 
