244 ON ACCIDENTAL OE SUBJECTIVE COLOES 



tion a little deviating from the line of distinct vision, the eye perceives, in gen- 

 eral, a mass of reddish light around the candle, and, in this light, as on a back- 

 ground, may be seen the ramifications of the sanguineous vessels of the retina, 

 the base of the optic nerve and the foramen centrale. 



Sir D. Brewster holds it to be the better opinion that the light which sur- 

 rounds the candle is reflected on the retina by the inner concave surface, whether 

 of the crystalline or the cornea, and that the objects are, in one way or another, 

 amplified by the concave surfaces. His own opinion on the subject is that the 

 light is propagated from the luminous image of the candle, and that the retina, 

 in contact with the blood-vessels, is insensible to the propagated light, although 

 sensible to the direct light ; and consequently the blood-vessels would be delin- 

 eated in dark lines. As the retina does not extend over the foramen centrale, it 

 would at this point naturally present a black spot, and, as to the confused vision 

 of .the optic nerve, the latter appears less luminous than the surrounding retina. 



M. Wheatstone, who has varied this experiment in different ways, says that 

 it succeeds best in a dark chamber. When, one of the eyes being withdrawn 

 from the action of the light, the flame of a candle is placed beside the unshielded 

 eye, so as not to occupy the least of the central part of the field of vision, then, 

 so long as the candle is at rest, nothing is observed but a diminution of the sen- 

 sibility of the retina for the light ; but when the flame is moved, by raising or 

 lowering it within a small limit, for a space of time which varies with the sen- 

 sibility of the individual on whom the experiment is tried, the phenomenon pre- 

 sents itself spontaneously. The blood-vessels of the retina, M'ith all their rami- 

 fications, exactly as they are represented in the plates of Sommering, distinctly 

 show themselves, and are projected apparently on a plane before the eye, with 

 amplified dimensions. The image continues manifest only while the light is in 

 motion, and disappears immediately or soon after it has passed to a state of 

 rest. 



M. Wheatstone does not accept the ingenious explanation of this appearance 

 offered by Sir D. Brewster ; he thinks there is no difficulty in accounting for 

 the image ; that it is evidently the shadow resulting from the obstruction of the 

 light by the blood-vessels spread over the retina. The real difficulty was in 

 explaining why this shadow is not always visible. To account for this, M. 

 Wheatstone advances different facts which tend to prove that an object more or 

 less brilliant than the ground on which it is placed, becomes invisible when it is 

 continually presented to the same point of the retina, and the rajndity of its dis- 

 appearance is so much the greater as the difference of the luminous intensities 

 between the object and the background is less ; but, by changing continually the 

 flace of the image of the object on the retina, or by causing it to act intcrmit- 

 tingly on the same point, the object may be rendered visible in a permanent 

 manner. 



Applying this explanation to the phenomenon in question, M. Wheatstone 

 observes that every time the flame of the candle changes its place, the shadows 

 of the vessels fall on different points of the retina, which is evident from the 

 movement of the image while the eye remains at rest, a movement which is 

 always in a contrary direction to that of the flame : hence the shadow, by thus 

 changing its place on the retina, remains, according to the law previously estab- 

 lished, visible in a permanent manner ; but when the flame is at rest, the shadow 

 also is at rest, and consequently disappears. 



M. Wheatstone has contrived an instrument for showing a singular modification 

 of this experiment. It consists of a circular concave plate of metal, about two 

 inches in diameter, blackened on the exterior face, and perforated at its centre 

 with a hole nearly equal to the bore of a light gun ; on the interior face is fixed 

 a similar concave plate of roughened glass. By placing the opening between the 

 eye and the flame of a candle and setting the plate in motion, so as continually to 

 displace the image of the opening upon the retina, the blood-vessels will be seen 



