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time, upon a window, in the evening or on a dark day, and upon closing your eyes, and covering them with your hands, an exact delineation of the window will appear; but if any light be admitted through the fingers or laterally, the dark parts of the window will appear luminous or white, and vice versa, in a few seconds, when the retina becomes sensible to the lesser impressions of light penetrating the eyelids. This experiment requires some practice and calmness, without any perturbation of the spirits, or fatigue and uncasiness of the organ of vision to make it succeed well.
These appearances may be termed direct ocular spectra, which resemble their objects in colour as well as in form, and are produced by a slight irritation. A moderate stimulus excites the retina into action, which quickly ceases, while a greater stimulus excites it into a spasmodic action, which ceases and recurs alternately. And hence the appearance of the window recurs repeatedly before it entirely vanishes. In like manner, upon looking at the setting sun, so as not greatly to fatigue.the eye, a yellow spectrum is seen to appear and disappear several times, before it vanishes entirely; but if it be made to fall upon the grass or other coloured objects, it will change its colour by an intermixture with theirs.
OF REVERSE OCULAR SPECTRA.
BUT besides these ocular spectra, which represent the form and colour of their objects, there are others which represent the form, but a colour the reverse of their objects.
Sir Isaac Newton observed, that the thin plates of air between the convex and plane glasses which transmitted the violet, reflected the yellow rays, those which
reflected red transmitted a bluish green, and those which reflected orange transmitted blue. These colours may therefore be denominated the reverse of each other.
Hence a red object produces a green spectrum, an orange produces a blue spectrum, and a yellow object produces a violet spectrum, and vice versa, when the retina having been stimulated by violent impressions of the rays of light falls into spasmodic action of an opposite kind; as when any of our muscles have been long or violently extended, and the action ceases, the limb is stretched the contrary way by the antagonist muscles, or falls down motionless, or in a temporary paralysis. Place a piece of coloured silk upon a sheet of white paper about half a yard from the eye, and look steadily at it for a minute; then, upon moving the eyes to another part of the paper, or closing them and putting the hands before them so as to admit some light, but not too much, to enter the eyes through the eyelids; a spectrum will appear of an opposite colour, namely, such as would result from a combination of all the other prismatic colours. For, as the direct spectrum, without any lateral light, is an evanescent representation of the form and colour of its object, in the unfatigued eye, but partaking of the colour of circumambient objects, when mixed with lateral light from them; so the reverse spectrum, without lateral light, is a representation of the form of the object, in the fatigued eye, with a colour compounded of all the prismatic colours, excepting that of the object; and with lateral light, the colour of it is compounded with the colour of the direct spectrum of the circumjacent objects, in proportion to their respective quantity and brilliancy. To prove that the green colour of the reversed
a spectrum, arising from a red object, is a compound of ball the other prismatic colours, excepting that of the object; let the prismatic colours be laid upon a circular wheel of pasteboard of four inches diameter in this proportion, red 45°, orange 27°, yellow 48°, green, 60°, blue 60', indigo 40°, and violet 80°, and the wheel, upon being turned quickly on its axis, will appear white, the compound of all the prismatic colours, as has been formerly observed. But now let the red compartment be entirely left out, and the others be proportionably enlarged to complete the circle: then as the orange is a mixture of red and yellow, the adjacent colours, substitute yellow in its place; and as the violet is a mixture of red and indigo, its adjacent colours, substitute indigo in its place; that all red may be entirely excluded from the painted wheel, as the fatigued eye is supposed to become insensible to the impressions of the red rays, when the green spectrum is excited in it. Now upon turning this wheel quickly upon its axis, a green colour is produced corresponding with great exactness to the green colour of the reversed spectrum obtained in the fatigued eye from a red object.
In contemplating these reverse spectra, they are found to appear and disappear several times successively before they vanish, and when disappearing they may be renewed by withdrawing the hands from before the closed eyes, and admitting a little light through the eyelids. Because the retina, being sensible to all other rays, except those with which it was lately fatigued, is at the same time stimulated into such motions as produce the reverse spectra. On looking on the meridian sun as long as the eyes can bear its brightness, the disk will become pale and then blue,
surrounded with a white halo, and upon closing the eyes, the spectrum will appear yellow at first, and soon change into blue, which may continue for some days; as the retina having been so violently stimulated into action falls into various successive spasmodic actions, which may continue several hours, or even induce a temporary paralysis of the organ of vision. Hence the danger of looking too long at very bright and luminous objects.
The direct and inverse spectra may exist at the same time, excited by different or by the same objects. When six inches square of bright Indian pink paper is viewed for some time, upon a foot square of white paper, the green spectrum, viz. the reverse of the pink paper, appears in the center, while the external spectrum is of a pink colour, which is the direct spectrum of the coloured paper. The same will happen when the internal area is white, and the external area is pink colour. While the rays, from the bright internal object, fall with their full force upon the retina, and by fatiguing it, induce the reverse spectrum; many scattered rays from the internal pink coloured paper, fall upon the external parts of the retina, with such force as is sufficient to produce a direct spectrum of it. In the same manner, six inches square of violet paper, viewed on the same white ground, exhibited a yellow spectrum, the reverse of the violet, while the external spectrum was of the same colour, being its direct spectrum, produced as before explained.
If two colours, mutually the reverse of each other, be viewed, one surrounding the other, the scattered rays from the one, which only tend to excite its direct spectrum, coinciding with the reversed spectrum of the other, will make both the spectra more bright and
It is worth while to remember, that as the reverse spectrum of any coloured object is the result of a mixture of all the other prismatic colours in due proportion, it may indifferently be considered as their direct spectrum, as in reality it is both. Hence the dif ficulty of gaining a direct spectrum of any coloured object, in the day time, (when there is much lateral light from other objects,) except of very bright objects, such as the sun, or by looking through an opake tube; because the lateral light, falling upon the central part of the retina, contributes to induce the reverse spectrum, which is at the same time the direct spectrum of the lateral light, deducting only the colour of the central object. And for the same reason it is difficult to gain the reverse spectrum, where there is no lateral light to contribute to its formation. See Darwin's Exp. Phil. Tran. vol. 76, for 1786.
HAVING considered the limits of distinct vision with the naked eye, and found them to be very con tracted, we shall now inquire what advantages we may derive from optical instruments.
A single microscope is nothing else than a very convex lens, which magnifies the image of an object in the proportion of the focal distance of the lens to six inches; at which distance the naked eye has the most distinct vision. If the focus of the lens be one tenth of an inch, and an object be placed at that distance from it, the rays, after refraction through it, will proceed parallel; so that an eye placed any where in the axis of it, will have a distinct vision, and will see the image as large as if the eye were placed as near to the