495. Behind the partition, and directly facing each aperture, place two mirrors, enclosed in the wainscot, in an angle of 45°. These mirrors are each to be eighteen inches square: and all the space between them must be enclosed with pasteboard painted black, and well closed, that no light may enter: let there be also two curtains to cover them, which may be drawn aside at pleasure. When a person looks into one of these fictitious mirrors, instead of seeing his own face, he will see the object that is in front of the other; thus, if two persons stand at the same time before these mirrors, instead of each seeing himself, they will reciprocally see each other. There should be a sconce with a lighted candle, placed on each side of the two glasses in the wainscot, to enlighten the faces of the persons who look in them, or the experiment will not have so remarkable an effect.

496. Place a shilling in a large drinking glass of a conical shape, small at the bottom and wide at the top, and let it be half full of water; then place a plate on the top of it, and turn it quickly over, that the water may not escape. There will be seen on the plate a piece of coin the size of half-a-crown, and a little higher up, another the size of a shilling.

497. The multiplying-glass is a flat plano

h

A

k

C

convex lens, ab, cd, in the above figure. Rays issuing, from the object B, and falling upon the plane surface bc, come to the eye A in the direction ef, which show the object in its true place at B. But rays from the same object also fall upon the flat surfaces ab and c d, and thence coming to the eye in the directions h and k, make the object also to be seen at C and D, so that the object B will be multiplied into as many different objects as the lens has flat surfaces.

498. The following experiment was made by Mr. H. Phillips, to show the different effects of natural and artificial lights on plants. He selected plants of the mimosa, elegans, nova, and decurrens, while their pinnated leaves were fully expanded. On placing them in a dark room, the leaves immediately collapsed like the sticks of a fan, or as the feathers of a bird's wing fold over each other. The strongest artificial light that could now be thrown on them had no effect

on the automatic motion of the plants, and the foliage remained in a collapsed state until they were removed into the natural light of day, when their sensitive properties immediately became perceptible, and the whole of the leaflets were seen moving towards their natural and elegant direction, with as much regularity as a regiment of soldiers file off at the word of command.

499. Dr. Brewster's mode of arranging the plane mirrors in the caleidoscope is exceedingly perfect. In the ordinary form it is, however, still confined to the gaze of the individual whose eye is placed in contact with the tube; and we now furnish a sketch illustrative of the magic lantern caleidoscope, by which it may be exhibited in a large room.

500. The general principle of the apparatus requisite for this subject is shown in fig. 2, where CDGF is the tube containing the reflectors A, O, E, &c. The objects from which the pictures are to be created are placed in the cell CD, which may be made either to have a rotatory movement round the axis of the tube, or to slide through a groove, like the sliders of a magic lantern. These objects are powerfully illuminated by a lens B, which concentrates upon them the direct light of the lamp or candle H; and also the part of the light which is reflected from the mirror MN. At the eye end E of the caleidoscope is placed a lens LL, close to the end of the reflectors, and having its centre coincident with the centre of the aperture at E. In order that this lens may form behind it an image, PP, of the objects placed in the object-plate CD, its focal length must be less than the length A E of the plates. If the focal length of L L is so small as one half of A E, then it follows, from the principles of optics, that the distance LP, at which the image is formed behind the lens, will be precisely equal to the distance A E of the object; but this is obviously too small a distance, for the diameter of PP would be equal only to the apparent diameter of the circular aperture of the caleidoscope, or to twice AO. Hence it is necessary that the focal length of the lens L L be less than A E, and greater than half of AE. Two-thirds or three-fourths of A E will be found to be a suitable focal strength; for, if it is larger than this, the image will be formed upon the wall at too great a distance from the instrument.

501. When the instrument is thus fitted up, an enlarged image of the pattern will be thrown upon the wall, which must be covered with white paper, or some white ground, in order to exhibit the colors to advantage. By turning the object-plate round its centre, or, if it is a rectilineal one, by pushing it through the groove, and at the same time giving it a rotatory motion, the pattern on the wall will undergo every possible transformation, and exhibit to the spectators, in a magnified form, all those variations which would have been observed by applying the eye to the caleidoscope.

502. If a box be formed with a small aperture in one of its sides, and three lights be made to revolve on a piece of wood in the centre, the candle opposite the axis will be visible on a

semi-transparent screen intended to receive its image. The other two candles will also have their flames depicted on the screen, at similar distances, on each side. This experiment serves to show the exceeding minuteness of the rays which emanate from the candles: for, if they were not exceedingly small, they could not pass each other without destroying the perfection of the image.

503. This experiment may be made very amusing by placing a piece of glass, with a ship or any other object painted on it, in the aperture; and, when the candles are parallel to the screen, three images will be seen; but, on making them revolve through a quarter of a circle, only one image will be visible on the screen. By increasing the number of candles the ships will be increased in an equal ratio; and a single ship, by turning the row of candles, will be converted into an entire fleet.

504. If a person be placed directly before a large concave mirror, but further from it than its centre of concavity, he will see an inverted image of himself in the air, between him and the mirror, of a less size than his own person. If he hold out his hand towards the mirror, the hand of the image will come out towards his hand, and coincide with it, of an equal bulk when his hand is in the centre of concavity; and he will imagine he may shake hands with his image. If he reach his hand further, the hand of the image will pass by his hand, and come between it and his body; and, if he move his hand towards either side, the hand of the image will move towards the other; so that whatever way the object moves the image will move the contrary way. A bystander will see nothing of the image, because none of the reflected rays that form it enter his eyes.

505. From this remarkable property of a concave mirror to form an image in the air, mirrors of this sort are used to produce a variety of singular appearances, to amuse the curious, or to impose upon the ignorant and superstitious. To a few we shall give a place. If a fire be made in a large room, and a smooth mahogany table be placed at a considerable distance, near the wall, before a large concave mirror, so situated that the light of the fire may be reflected from the mirror to its focus upon the table; if a person stand by the table he will see nothing upon it but a longish beam of light; but, if he stand at a distance towards the fire, not directly between the fire and the mirror, he will see an image of the fire upon the table, large and erect. If another person, who knows nothing of the experiment beforehand, should chance to come into the room, and should look from the fire towards the table, he would be startled at the appearance; for the table would seem to be on fire. In this experiment there should be no light in the room but what proceeds from the fire; and the mirror ought to be at least fifteen inches in diameter.

506. If the fire used in the last experiment be extinguished or covered by a screen, and a large candle be placed in a similar position, a person standing by the candle will see the appearance of a star, or rather planet, upon the table, as brilliant as Venus or Jupiter in a cloudless sky.

If a slender wax-taper be placed near the candle, a satellite to the planet will appear on the table; and, if the taper be moved round the candle, the mimic satellite will go round the planet.

507. If a person looks for some minutes steadily on a window, in the beginning of the evening twilight, or in a dark day, and then moves his eyes a little, so that those parts of the retina on which the dark frame-work of the window was delineated may now fall on the glass part of it, many luminous lines, representing the framework, will appear to lie across the glass panes : for those parts of the retina which were before least stimulated by the dark frame-work are now more sensible to light than the other parts of the retina which were exposed to the more luminous parts of the window.

508. Make with ink, on white paper, a very black spot, about half an inch in diameter, with a tail about an inch in length, so as to represent a tadpole; look steadily for a minute on this spot, and, on moving the eye a little, the figure of the tadpole will be seen on the white part of the paper, which figure of the tadpole will appear whiter or more luminous than the other parts of the white paper; for the part of the retina on which the tadpole was delineated is now more sensible to light than the other parts of it which were exposed to the white paper. This experiment is mentioned by Dr. Irwin, but is not by him ascribed to the true cause, namely, the great sensibility of that part of the retina which has been exposed to the black spot, than of the other parts which had received the white field of paper.

509. Fig. 3 will serve to illustrate the construction of a very elegant optical toy, or hydromantic temple.

510. Let us suppose two vessels, the uppermost filled with water, and sustained by two little pillars-one of which is hollow, and furnished with a cock, e. Let the other vessel be divided by a partition into two parts; the lower of which may be opened or closed by means of a cock at e. Upon the partition place an image, a, which the spectator at ƒ cannot see by a direct ray, f, g. If now the cock, e, be opened, the water descending into the cavity, beneath the ray, will be refracted from the perpendicular; so that the spectator will now see the object by the refracted ray. And again shutting the cock e, and opening the other d, the water will descend into the lower cavity; whence, the refraction ceasing, no rays will now come from the object to the eye; but, shutting the cock d again, and opening the other e, the water will fill the cavity again, and bring the object in sight afresh.

510.* Provide a sufficient number of small equilateral prisms, a few lines only in breadth, and in length equal to the height of the painting which you intend to make, and place them all close to each other on the ground to be occupied by the painting. Then cut the painting into bands equal to each of the faces of the prism, and cement them, in order, to the faces of the same side. When this is done, take a painting quite different from the former, and, having divided it into bands in the same manner, cement them to the faces of the opposite side. It

is hence evident, that when on one side you can see only the faces of the prism turned towards that side, one of the paintings will be seen; and, if the picture be looked at on the opposite side, the first will disappear, and the second only will be seen. A painting may even be made which, when seen in front, and on the sides, shall exhibit three different subjects. For this purpose, the picture of the ground must be cut into bands, and be cemented to that ground in such a manner that a space shall be left between them, equal to the thickness of a very fine card. On these intervals raise, in a direction perpendicular to the ground, bands of the same card, nearly equal in height to the interval between them; and, on the right faces of these pieces of card, cement the parts of a second painting, also cut into bands. In the last place, cement the parts of a third picture, cut in the same manner, on the left faces of the pieces of card. It is evident that when this picture is viewed in front, at a certain distance, the bottom painting only will be seen; but if you stand on one side, in such a manner that the height of the slips of card conceals from you the bottom, you will see only the picture cemented in detached portions to the faces turned round towards that side; if you move on the other side, a third painting will

At the distance BD the arrow ABC appears' or is seen by the eye D, under the angle ADC. At the distance BE, double that of BD, the arrow is seen under the angle A EC, which is but half the quantity of AD C. And at the distance BF, triple the distance BD, the same arrow is seen under the angle AFC, which is but a third part of the quantity of the angle ADC. And therefore the arrow must appear but half as long when seen from E as from D, and but a third part so long when seen from F as from D.

512. Fix, at the height of the eye, on a dark ground, a small round piece of white paper, and a little lower, at the distance of two feet to the right, fix up another, of about three inches in diameter; then place yourself opposite to the first piece of paper, and, having shut the left eye, retire backwards, keeping your eye still fixed on the first object; when you are at the distance of nine or ten feet, the second will en-, tirely disappear from your sight.

513. If you place yourself before a concave mirror, at a proper distance, your figure will appear inverted; and, if you stretch out your hand towards the mirror, your will perceive anothe

hand stretched out to join and meet it, though not perceptible to the touch. But if a sword be substituted for the hand, and your micror is a foot or more in diameter, another sword will appear, and seem to encounter with that in the hand. The larger the mirror is, the more perfect will be the illusion.

514. Affix to a dark wall a round piece of paper, an inch or two in diameter; and a little lower, at the distance of two feet on each side, make two marks; then place yourself directly opposite to the paper, and hold the end of your finger before your face in such a manner that, when the right eye is open, it shall conceal the mark on the left, and, when the left eye is open, the mark on the right: if you then look with both eyes to the end of your finger, the paper, which is not at all concealed by it from either of your eyes, will nevertheless disappear.

515. The general effect of the camera obscura is the same as may often be observed in a dark room, where there is a small hole in the window-shutter; the great masses of light and shade before the windows, being represented in an inverted position in the parts of the room diametrically opposite to them, which are illuminated in different degrees, according to the quantity of light which can reach them in straight lines from the external objects. A lens, of a focal length somewhat smaller than the distance of the surface on which the picture is projected, renders the images much more distinct; but some of them are unavoidably imperfect and ill defined, unless the objects happen to be situated at the same distance from the aperture; for the focus of the lens can never be adjusted at once to nearer and more remote objects; nor would the picture be rendered more natural by such an adjustment, for it would present to the eye at one view, with equal distinctness, objects which never can be seen at once without some degree of confusion. Sometimes. the picture is intercepted by a speculum placed obliquely, and is thrown upwards on the surface of a plate of ground glass, upon which its outline may be traced with a black-lead pencil, and an impression may be taken from it on moist paper, which will represent the natural situation of the objects without inversion. Another arrangement is, to place the lens horizontally, with the speculum above it, which throws the image. through the lens, upon a flat surface placed be low, on which the objects may be delineated in their natural position, but not without some impediment from the interception of the light by the hand and the instrument employed. Such a surface, however, ought not to be perfectly flat, in order to afford the most distinct image; although by means of a meniscus-lens, with a cover admitting the light only through a small aperture near its centre, on the principal of Dr. Wollaston's periscopic spectacles, an image nearly flat might be obtained; but in this case too much of the light would be excluded. It has been usual to consider the image of a very distant object, formed by a convex lens, as a portion of a spherical surface, of which every part is equally distant from the centre of the lens; but this estimate is extremely erroneous, for the effect of the obliquity of the different

rays, as a bc, def, g h i, issuing from all points of the object AB, on the side next the convex lens EF, after passing through the lens, will converge to as many points beyond it, and at those points of convergent meeting they will form an inverted picture C D of the object on a white paper.

517. Among the various phenomena of vision which were observed by the philosophers of the last century those which arise from indirect impressions, and from the influence of highly attenuated light upon the retina, seem to have escaped their

notice.

518. If we look at a narrow slip of white paper placed upon a black or a colored ground, it will never appear to vanish, however long and attentively we view it. But if the eye is fixed steadily upon any object within two or three inches of the paper, so as to see it only indirectly, or by oblique division, the slip of paper will occasionally disappear, as if it had been entirely removed from the ground-the color of the ground extending itself over the part of the retina occupied by the image of the slip of paper.

519. If the object seen indirectly is a black stripe on a white ground, it vanishes in a similar manner; and, what is still more remarkable, the same phenomena of disappearance take place, whether the object is viewed with one or with both eyes.

520. When the indirect object is luminous, like a candle, it never vanishes entirely, unless it is placed at a great distance, but it swells and contracts, and is surrounded by a halo of nebulous light, so that the excitement must extend itself to contiguous portions of the retina, which are not influenced by the light itself.

521. If we place two candles at the distance of about eight or ten feet from the eye, and about twelve inches from each other, and view

the one directly, and the other indirectly, the indirect image will be encircled with a bright ring of yellow light, and the bright line within the ring will have a pale blue color. If the candles are viewed through a prism, the red and green lights of the indirect image vanish, and leave only a large mass of yellow, terminated with a portion of blue light.

522. While Dr. Brewster was performing this experiment, and looking steadily and directly at one of the prismatic images of the candle, he was surprised to observe that the red and green rays began to disappear, leaving only yellow and a small portion of blue, and, when the eye was kept immovably fixed on the same part of the image, the yellow light became almost pure white, so that the prismatic image was converted into an elongated image of white light.

523. If the slip of white paper, viewed indirectly with both eyes, is placed so near as to be seen double, the rays which proceed from it no longer fall on corresponding points of the retina. In this case the two images do not vanish simultaneously; but, when the one begins to disappear, the other begins soon after it, so that they sometimes appear to be extinguished at the same time.

524. In order to ascertain whether or not the accidental color of an object seen indirectly would remain after the object itself had disappeared, he placed a rectangular piece of a red wafer upon a white ground, and, having looked steadily at an object in its vicinity, the wafer disappeared, and, though the accidental color showed itself just before the wafer vanished, yet

no trace of color was visible afterwards.

525. The insensibility of the retina to indirect impressions, has a singular counterpart in its insensibility to the direct impressions of attenuated light. When the eye is steadily directed to objects illuminated by a feeble gleam of light, it is thrown into a condition nearly as painful as that which arises from an excess of splendor. A sort of remission takes place in the conveyance of the impressions along the nervous membrane; the object naturally disappears, and the eye is agitated by the recurrence of excitements which are too feeble for the performance of its functions. If the eye had, under such a twilight, been making unavailing efforts to read, or to examine a minute object, the pain which it suffers would admit of an easy explanation; but, in the present case, it is the passive recipient of attenuated light, and the uneasiness which it experiences can arise only from the recurring failures in the retina to transmit its impressions to the optic nerve.

526. The preceding facts respecting the affections of the retina, while they throw considerable light on the functions of that membrane, may serve to explain some of those phenomena of the evanescence and reappearance of objects, and of the change of shape of inanimate objects, which have been ascribed by the vulgar to supernatural causes, and by philosophers to the activity of the imagination. If in a dark night, for example, we unexpectedly obtain a glimpse of any object, either in motion or at rest, we are naturally anxious to ascertain what it is, and our curiosity calls forth all our powers of vision. This anxiety,