NATURAL SCIENCES OF PHILADELPHIA. 
19 
Remarks on an Optical Illusion. 
BY ALEXANDER WILCOCKS, M. D. 
I desire to draw the attention of the Academy to one of the phenomena oi 
sight, which is interesting, because I conceive that it has led some zealous in- 
vestigators of the mysteries of nature into error. 
I allude to that power of the retina, by which an impression made upon it by 
a luminous object is retained, during an appreciable time. 
The most familiar illustration of this law, is in the whirling of a piece ot 
lighted charcoal in the dark. The result is the formation of a luminous ring, 
more or less perfect, according to the rapidity of the motion of the hand. 
Experience has not only abundantly proved the existence of this peculiarity 
of the retina ; but it has also determined the duration of the impressions. It has 
been found to be from one tenth, to one eighth of a second. 
Such at least is true of persons in good health ; but there is reason to believe 
that there are cases where the duration of impressions is much longer. 
If when under suitable circumstances, a luminous body is moved before the 
eye at rest, we perceive a line of light, we may naturally expect, that when the 
converse condition obtains, we shall witness a similar phenomenon. 
By the converse condition, I mean that wherein the luminous object is at rest, 
and the eye in motion about one of its own axes. 
Requirements so simple are easily obtained ; the most convenient object 
upon which to make the experiment is the star Sirius. The precautions to be 
observed, are to choose a time when the moon does not shine, and the star is 
high above the horizon ; and a place where the eye will not be dazzled by any 
artificial light. The observer must be free from all sense of fatigue, because the 
eye sympathizes readily with the condition of the body, and any cause that in- 
terferes with the rapidity of the motions of the organ, would endanger the 
success of the experiment. 
Experiment 1st. Let the observer turn his face toward the star, but fix his 
eyes upon a point about ten degrees to the east of that object, then suddenly 
placing his eyes upon the star, he will perceive a ray or flash of light reaching 
from the star to a point ten degrees to the west. 
Experiment 2nd. Keeping his eyes for a moment upon the star, let the ob- 
server return them to their first position, and he will perceive that the flash ot 
light will extend from the star to that position. 
Any one who will trace in his mind the course pursued by the image of the 
star upon the retinae, during this motion of the eyes, will ascertain that these 
results are precisely what d, priori reasoning should lead us to expect. 
In the experiments just described, the flashes of light are straight. If instead 
of the star Sirius, we choose for our experiment, the planet Jupiter, at a time 
when that body is within twenty-five degrees of the zenith, we shall discover 
that the flashes of light are curved, and the convexity of the curve invariably 
toward the earth. 
It must be noticed in connection with this peculiarity, that any observations 
upon an object at such an altitude as the one proposed, viz. sixty-five degrees, 
would require that the eyes should be much raised above their usual plane of 
motion. The physiologist may find much difficulty in explaining why the eleva- 
tion of the eyes in their orbits should cause the flash of light to be curved ; 
and why the convexity should be toward the earth ; but, that the facts exist, 
others may verify for themselves. 
The particular interest which attaches to this experiment is from the light which 
it throws upon the results of some observations upon meteors, the explanation 
of which has baffled the ingenuity of astronomers. 
In the year 1798, Brandes, of Leipsig and Benzenberg, of Dusseldorf, under- 
took to measure the height and velocities of meteors. Having established a base 
line of 46,200 feet, and provided themselves with chronometers, they stationed 
1858.] 
