168 EXPERIMENTS BY C. TOMLINSON, ESQ. 
2nd, C4 H4 + 0 oxideof binolefiant gas. 
C4 4" 02 aldehyde. 
4- 0^ aldehydic acid. 
-J- O^ hydrous acetic acid. 
ON THE action OF FLASHES 
LIGHT UPON RAPIDLY ROTATING 
disks. 
Bt Charles Tomlinson, Esq, 
Professor Wheatstone, I believe, first 
announced the beautiful fact “ that a rapidly 
moving wheel, or a revolving disk on which 
any object is painted, seems perfectly station- 
ary when illuminated by the explosion of the 
electric jar.” 
This experiment is adduced by Mr. Wheat- 
stone, to shew that the duration of electric 
light embraces a point of time so extremely 
minute that the revolving wheel, or disk, has 
not time to pass through any perceptible space, 
and that, therefore, it appears, during the illu- 
mination. stationary; I find, however, that 
the effect is not confined to electricity, but 
may be produced by any very sudden flash of 
light. 
Of the disks that I employed I need only 
mention two ; The first, six inches in diameter, 
was divided into sixteen parts, painted, alter- 
nately, red and black; on the second disk, 
of the same size, were painted in large cha- 
racters the words, at rest on white ground. 
Both disks were connected with a small mul- 
tiplying arrangement. 
The effects can be produced with phos- 
phuretled hydrogen, exhibited in bubbles from 
phosphuret of lime, in water. When the bub- 
bles come up slowly without interrupting each 
other, both disks appear, stationary during 
rotation ; but when the bubbles come up too 
quickly, the black and red spaces exhibit a 
dancing sort of motion, and sometimes two 
black spaces seem joined into one, to the 
exclusion of the intervening red, and vice versa; 
so also with the second disk, the words cross 
each other in various directions when the 
flashes of light interfere with each other; and. in 
both cases, confusions, of course, excited 
when an impression is made on the retina 
before succeeding impressions have departed. 
Similar confused effects are produced with 
a stream of electricity instead of the discharge ; 
as also by the rapid succession of sparks from 
a magnet, but in any case when the flash of 
light is distinct and sudden, the effect is 
complete. 
Soap bubbles, blown with hydrogen or the 
mixed gases, and fired by means of a filament 
of cotton passed through a small tube, and 
wetted with alcohol ; gunpowder, done up in 
the form of a boy’s cracker ; fulnsinate of mer- 
cury struck on an anvil, may all be success- 
fully employed. 
These experiments were performed in a 
darkened room, not of necessity, but the results 
are best observed in this manner. In 
Mr. Wheatstone’s experiment, the presence 
of light, either natural or artificial, does not 
interfere with its success. 
The experiment may be made to succeed 
by the flame of a lamp or candle. In order 
to effect this 1 employed a disk of pasteboard, 
twelve or thirteen inches in diameter, with 
a narrow slit cut out, extending from the 
centre nearly to the circumference, and con- 
nected with a multiplying arrangement. The 
light of the lamp was condensed by a lens, and 
thrown upon the back of the slitted disk, and 
the black and red disk placed in the front of 
the former, so as to receive a flash of light from 
the lamp every time the slitted disk performed 
one revolution. Op causing both disks to 
revolve, the black and red spaces were dis- 
tinctly brought out, assuming, however, a 
curved form. 
But, perhaps, the most convenient method 
of producing this phenomenon, is to stand 
behind the slitted disk, while in front of it, at 
the distance of two or three feet, the radi- 
ated disk is made to rotate. On rotating the 
slitted disk the effect is very complete. The 
radii are, however, curved either upwards or 
downvmrds, according as the eye of the observer 
is above or below the axis of the disk, except 
the radii which, for the time being, are 
vertical to the axis above and below, and these 
are not curved. This effect takes place when 
the disks are revolving in the same direction. 
The order will be inverted, if the disks move in 
opposite directions; a change also will take 
place in the direction of the curvature of the 
radii, according to the angle at which the eye 
is placed. 
This experiment is somewhat analogous to 
one by Dr. Roget, “ when a carriage wheel, 
rolling along the ground, is viewed through the 
intervals of a series of vertical bars, such as 
those of a palisade, or of a Venetian window 
blind. Under these circumstances, the spokes 
of the wheel, instead of appearing straight, 
as they would naturally do if no bars inter- 
vened, seem to have a considerable degree of 
curvature." (Phil. Trans. 1825.) 
It was found that " the velocity of the wheel 
must not be so great as to prevent the eye 
from following the spokes as they revolve.” 
So that Dr. Roget’sexperiment relates simply 
to the curvature of thespokes of a wheel seen 
through a narrow aperture ; and he accounts 
for this fact by assuming the deception to arise 
from separate parts only of each spoke being 
seen at the same moment; the remaining 
parts being concealed from view by the bars. 
He also found that " when tlie disk of the 
wheel, instead of being marked by a number 
of radiant lines, has only one radius marked 
upon it, it presents the appearance, when 
roiled behind the bars, of a number of radii, 
each having the curvature corresponding to 
its situation, their number being determined 
by that of the bars which intervene between 
the wheel and the eye. So that it is evident 
that the several portions of one and the same 
line, seen through the intervals of the bars 
form on the retina, the images of so many dif' 
ferent radii,” 
My experiment differs from that of Dr- 
Roget, inasmuch, that the red and black disk 
may be made to revolve with very great rapid- 
ity, by which the black is lost to the eye, 
and the red alone reflected, slightly diluted 
with black. '1 he effect of viewing this disk 
during rotation through the rotating slitted 
disk, is to decompose the former, and presen 
