552 
SECOND REPORT—1832. 
merring, are distinctly seen, apparently projected on a plane be¬ 
fore the eye, and greatly magnified. The image continues only 
while the flame is in motion ; directly, or soon after, the flame be¬ 
comes stationary, it dissolves into fragments and disappears. 
Mr. Wheatstone dissented from the ingenious explanation of 
this appearance offered by Sir David Brewster, and also from 
that opinion stated to be the generally received one ; and beg¬ 
ged to repeat the solution he had published, and which he had 
not since been induced to relinquish. Mr. W. observed, that 
there was no difficulty in accounting for the image ; it evident¬ 
ly was a shadow resulting from the obstruction of light by the 
blood-vessels spread over the retina ; the real difficulty was to 
explain why this shadow is not always visible. To account for 
this, Mr. W. adduced several facts, which tended to prove that 
an object, either more or less luminous than the ground on which 
it is placed, when continuously presented to the same point of the 
retina, becomes invisible; and the rapidity of its disappearance 
is greater as the difference of luminous intensity between the ob¬ 
ject and the ground is less; but by continually shifting the place 
of the image of the object on the retina, or by making it act in¬ 
termittently on the same point, the object may be rendered per¬ 
manently visible. To apply this explanation to the phenomenon 
in question, Mr. W. observed, that whenever the flame of the 
candle changes its place, the shadows of the vessels fall on dif¬ 
ferent parts of the retina ; which is evident from the motion of 
the figure while the eye remains still, which is always in a con¬ 
trary direction to that of the flame. Hence the shadow, being 
thus made to change its place on the retina, remains, according 
to the law above stated, permanently visible; but instantly the 
flame is at rest, the shadow also becomes stationary, and con¬ 
sequently disappears. 
Mr. Wheatstone then exhibited an instrument for showing 
an original variation of this experiment: it consisted of a cir¬ 
cular plate of metal, about two inches in diameter, blackened 
at its outer side, and perforated at its centre with an aperture 
about as large as an ordinary gun-hole ; to the inner face was 
fixed a similar plate of ground glass. On placing the aperture 
between the eye and the flame of a candle, and keeping the 
plate in motion, so as to displace continually the image of the 
aperture on the retina, the blood-vessels will be seen distri¬ 
buted as before, but will now appear brighter, and the spaces 
between the ramifications will be seen filled with innumerable 
minute vessels, anastomosing with each other in every direction, 
which were invisible in the former experiment. In the very 
centre of the field of vision there is a small circular space, in 
