Page:Spectropia, or, Surprising spectral illusions.djvu/15
have the power of appreciating the image there formed, and convey it up through the ultimate parts of the retina (Fig. 2), thence along the optic nerve fibres to the brain. We are inclined to regard the extremities of the rods and cones as the true seat of perception, in consequence of observing a considerable distance between the retinal blood-vessels and the choroid, when performing Parkinje's experiment, This experiment consists in passing a lighted candle slowly to and fro before the eyes, at about two or three inches from the nose, when the retinal vessels will exhibit themselves before the observer not unlike branching trees.
They may be seen by daylight, by passing the large teeth of an ordinary comb slowly backwards and forwards before the eye whilst looking on a smooth sheet of paper, or upon the sky. Fig. 4 represents those of the left eye, as seen by candlelight. The spot marked k is the exact centre of the retina. (The same letter marks the same spot in Fig. 1.) It is the seat of most distinct vision, j is the entrance of the optic nerve (Figs. 4 and 1), from the centre of which the retinal artery will be seen emerging and spreading over the entire retina; but in the diagram that part only is represented which could be seen tolerably distinct. The background to the artery appears of a pale red, except at the part occupied by the optic nerve, where it is white.
After this rapid glance at so complicated a structure, and bearing in mind that some persons can see its several parts with vastly greater facility than others, it cannot be a matter of surprise that individuals not aware of these facts are, now and then—especially at night, and when carrying a light about—startled by what they fancy an apparition, but which is in reality nothing more than some part of the structures above considered.
A lady assures us that she saw the ghost of her husband as she was going downstairs with a lighted candle in her hand. The spot k, Fig. 4, when seen against a wall a few feet distant, appears about the size of a human head, and wants very little to furnish it with features. Figured paper on the wall, and a host of other things, may supply them, or even the retinal artery, which often lends body and limbs. (Fig. 5.)
- This distance can easily be perceived by getting an impression on the retina according to the "Directions," page 4, and then, on performing the above experiment, the arterial ramifications and the central spot will be distinctly perceived to move over the spectral figure.