Upon the whole, we have endeavoured to illustrate these several propositions, which seem worthy to be remembered; viz. That the apparent brightness of an object depends upon the quantity of light received from any point by the eye; that its distinctness arises from the accuracy of the collection of its several pencils of rays into a focus on the retina; that its apparent situation depends upon the last direction of the rays, when they enter the eye; that its apparent magnitude, at a known distance, depends upon the magnitude of the optic angle; and that we judge of the distance of an object, not only by all these, but also by experience and our other senses.

OF OCULAR SPECTRA.

FROM the stimulus which the retina receives, from the impressions of the rays of light upon it, all vision probably depends: and we are not to suppose, that the retina continues in a passive state, while vision is performed, but that it is stimulated into action, like the other organs of the animal body, by irritation from the impressions of the rays of light, especially as it consists of fibres intermixed with a considerable quantity of nervous medulla; as is evident from its appearance, being easily torn asunder when immersed in warm water. Besides, being excited into action by irritation, in common with the muscles, they are alike fatigued or strengthened by action, are alike pained when stimulated into action in a state of inflammation, and alike subject to torpor and the effects of old age. The impressions, therefore, made on the retina, by the rays of light issuing from external objects, are the immediate causes of those sensations, which give us the knowledge of the existence of such objects. Nor do

these impressions wear off instantaneously upon the cessation of their cause, but continue for some time, like the effects of other irritations in the human body. The impressions made by a coal of fire whirled round in a circle, continue until the coal return to the place where its motion commenced, and renews the impression. When any one has attentively viewed a luminous object, for a few minutes, an image of it continues to be visible for some time, after closing his eyes, or removing them from the object; this may be called the ocular spectrum; which, in various circumstances, may be attended with different properties and phenomena, and which we shall endeavour to explain.

These ocular spectra may be different, according to the different degrees of irritability of the retina at the time, arising from the force of previous impres

sions.

1. The retina is not so easily excited into action by less irritation, when it has been lately subjected to greater. Hence, when a person enters a darkened room, after walking in the snow or bright sunshine, although the pupil soon expands, yet the retina will not so quickly become sensible to the weaker impressions of light in the room; and if the room be not too dark, the pupil may be observed to contract again as the sensibility of the retina returns. Look steadily, for the space of a minute, on a square piece of white paper placed upon a black ground, then move the eyes to a sheet of white paper, and a dark square will appear in the sheet of paper. The parts of the retina which received the image of the black ground not having been much irritated thereby, quickly become sensible of the greater impressions of the white sheet of paper, while the parts which have been more irritated by the vivid

impressions of light from the white square, are not so quickly excited into action after their fatigue.

In like manner, after looking at the sun, or any other luminous object, for a short time, that part of the retina which received his image becomes less sensible to smaller quantities of light, and therefore when our eyes are turned to less luminous parts of the sky, a dark spot is seen resembling the sun or other luminous object, which we beheld. This also seems to be the source of those muscæ volitantes, which some have mistaken for something floating in the aqueous humour of the eye. We endeavour to bring them into the center of the eye, that we may have a more distinct view of them, by the motion of our eye, but in vain, for they move with the eye, and have no other motion but what they receive from it. Hence, in the extreme debility of fevers, sick persons are observed to be employed in picking something from the bedclothes, occasioned by their mistaking these muscæ volitantes in their eyes, for realities. Again,

Place a square inch of white paper on a piece of strawcoloured silk, and having looked for some time steadily upon the paper, remove the eye to another part of the silk, and there will appear a yellow spectrum of the same figure with the paper on the silk, and of a deeper colour: for the central part of the retina, having been some time exposed to the stimulus of a greater quantity of white light, is now become less sensible of the lesser stimulus from the silk, and therefore sees only the yellow rays from the silk. Thus we cannot hear lower sounds after we have been accustomed to louder, nor will the stomach feel the stimulus of common food, which has been accustomed to the use of spirituous liquors,

2. But on the other hand, the retina is more easily excited into action by greater irritability, after having been exposed to less.

If the eyes be closed in a bright sunshine, and perfectly covered with a hat, for a few minutes, on the removal of the hat, the eyes being still closed, the retina becomes sensible of the red rays that penetrate the eyelids. A similar light is seen through the fingers exposed to the rays of the sun, or the light of a candle. In a dark day, or the beginning of the evening twilight, if you look steadily upon a window for a few minutes, and then move your eyes a little, so that those parts of the retina, which had before received the images of the wooden frame of the sashes, may now receive the images of the panes of glass, many luminous lines will now appear to lie across the panes. For those parts of the retina, which before were less stimulated by the dark frame work, are now more sensible to the light, than the other parts of it, which were exposed to a greater stimulus, from the lighter parts of the window. The same will happen also when the eyes are closed, by means of the small degree of light, which penetrates the eyelids. And if the frame be painted black, the image of it will be red: through the influence of the red rays, which more copiously enter the eyelids. In the same way, a black spot of any figure made on a white paper, will, upon moving the eye from it a little, appear upon the white paper, something whiter than the other parts of the paper, after the black spot has been steadily viewed for a few minutes. For, the parts of the retina, which received the image of the black spot, are now more sensible to the impressions of light, than the other parts, which received the picture of the white paper.

On closing the eyes, after viewing the black spot on the white paper, the spectrum will appear red, through the influence of the red rays that penetrate the eyelids, if in the day light, but yellow, if viewed by the light of a tallow candle: for tallow candles abound much with yellow light, which passes in greater quantities and force through the eyelids, than blue light: and hence the difficulty of distinguishing blue and green by this kind of light. In the same manner, the flood of light becomes intolerable to the eye that has been accustomed to slight irritations for some time past, upon emerging from darkness into open day, or into a room greatly illuminated. This will occasion an immediate contraction of the pupil, to counteract its force. The impressions made upon the retina continue for a short time, after the irritation ceases, and if continually renewed by the same cause, may be continued at pleasure: as in the case of a coal of fire whirled round in a circle, the luminous circle may be conti nued as long as the cause of the irritation acts: in the same manner, as the other senses retain the impres sions made upon them. When a hard body has been for some time pressed upon the palm of the hand, as in the tricks of legerdemain, it is not easy to distinguish, for a few seconds, whether it remain or be removed. The same may be said of taste.

Hence if you look, for half a minute, through a dark tube of two feet long, upon an area of half an inch of yellow silk, placed upon an inch area of blue silk, upon closing your eyes, the colours of the spectra will appear similar to the areas of the silk, provided the eye be not much fatigued with the quantity of light: but if the eye be kept too long upon them, the colours will be changed. Look steadily, for a short