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will be seen as at A, fig. 8, with its shadow on unpolished opaque body M, and place it either the right hand S of the cavity. As the candle C beside the hollow or in it, so that the body M, remains where it was, the observer instantly con- and its shadow m, may be distinctly seen by the cludes that what was formerly a cavity must now microscope, we shall have the appearance shown be a spherical elevation or segment of a sphere, in fig. 11, the elevation having sunk into a deas nothing but a raised body could have its sha- pression. This correction of the depression dow on the right hand S. If a second candle is arises from the introduction of a new illusion, now placed on the right hand side of A, so that namely, that which arises from the shadow m; it is between two candles, and is equally illumi- for it is evident that, as the body M appears to nated by both, the elevation will again sink into project its shadow in the direction M m, the lua cavity, as in fig. 9.
minous body must be supposed to be on the same 294. If the object A, in place of being a ca side D; and the evidence that this is the case is vity, is actually the raised segment of a solid more powerful than our knowledge that the cansphere, the same phenon
nomena will be observed, dle is actually at C, because it co-exists along the inverting eye-piece converting it into a with our perception of the depression A, whereas cavity. These iwo experiments may be made our knowledge of the situation of the candle is an most successfully with a seal, and an impression act of recollection. taken from it.
299. This correction of the delusion may be 295. It cannot therefore be doubted that the effected in another manner, which is perhaps optical illusion of the conversion of a cameo into more complete. If, in place of the unpolished an intaglio, and of an intaglio into a cameo, by body, we use a pin with a highly polished head, an inverting eye-piece, is the result of an opera as shown at M, fig. 12, and then apply the intion of our own minds, whereiy we judge of verting eye-piece, we shall have the effect shown the forms of bodies by the knowledge we have in fig. 13, the cavity A appearing depressed. acquired of light and shadow. The greater our The image s of the candle C, being seen by reknowledge is of this subject, the more readily flection in the polished head of the pin M, is seen does the illusion seize upon us ; while, if we are by the application of the eye-piece at s, on the but imperfectly acquainted with the effects of right hand side of M in fig. 13, so that we imlight and shadow, the more difficult is it to be mediately conceive, in opposition to our previous deceived. If the hollow is not polished, but knowledge, that the candle must be at D; and ground, and the surface round and of uniform hence the elevation falls into a depression the color and smoothness, almost every person, whe- moment the pin head is pushed up into the field ther young or old, will be subject to the illusion; of view. The shadow M m has also its influbut, if the object is the raised impression of a ence in the present case. seal upon wax, we have often found that, when 300. The next case in which this illusion is viewed with the eye-piece, it still seemed raised dispelled is when the sense of touch corrects the to the three youngest of six persons, while the three deduction formed through the medium of sight. eldest were subject to the deception. By such Let the cavity A be raised into an elevation by trifiling and often unappreciable circumstances the inverting eye-piece, as in fig. 8. Then if the is our judgment affected, that the same person at cavity is sufficiently deep, and if we place the one moment sees the convexity raised, and at point of our finger in the cavity, the evidence another time depressed, though viewed as nearly which this gives us of its being a depression is as possible under the same circumstances. This superior to the evidence of its being a cavity remarkable effect no doubt arises from the intro- arising from the inversion of the shadow; the duction of some casual reflected lights which the apparent elevation will of course sink into a deslightest change of position will produce. pression; but the moment the finger is with
296. Having thus seen how our judgment con- drawn it will again rise into an elevation. If cerning elevations and depressions is affected by the cavity is a long groove, the part not touched our degree of knowledge of the effects of light by the finger will appear elevated, while the part and shade, and by unappreciable causes, we shall touched by it will appear depressed. proceed to consider how our judgment is again 301. Having thus considered some of the deceived by the introduction of new substances. principal phenomena arising from the inversion
297. Let the depression A, illuminated by one of the object, we shall now proceed to explain candle as in fig. 7, be converted into an elevation some analogous facts which are owing to the as in fig. 8, by the application of an inverting semi-transparency of the body. If M N, fig. 14, eye-piece ; then if another candle C', fig. 9, is is a plate of mother-of-pearl, and A a cavity introduced so as to illuminate the depression A ground or turned in it; then if this cavity is illuin the same manner, and with nearly the same minated by a candle C, or by a window at C, in intensity as C does, the elevation will fall down place of their being a shadow at the side s, as into a depression. The cause of this is obvious: there would have been had the body been opaqu", the application of the inverting eye-piece pro- there is a quantity of refracted light seen alon; duces no effect whatever, for both the sides of the whole side s next the candle. The consethe cavity are syminetrically illuminated. In quence of this is that the cavity appears as an moving round the second candle C' from its po- elevation when seen only by the naked eye, as sition C', so as to stand beside C, it is curious to it is only an elevated surface that could have the observe the progress of the deception by which side s illuminated. The fact which we have now the depression is again changed into an elevation. stated is a very important one, in so far as it
298. If, when the depression A, fig. 10, is may affect the labors of the sculptor. In some converted into an elevation, we introduce a small kinds of marble the transparency is so great that
the depressions and elevations in the human face 306. An illusion of an analogous nature Dr. cannot be represented by it with any degree of Brewster once observed when looking at the abaccuracy; and consequently transparent marble bey church of Paisley, where the clustered ought never to be used for works of any import- columns of a Gothic pillar all sunk into hollow
flutings. The cause of this deception was not 302. Illusions arising from the same cause discovered, but it must have arisen from some may be observed even when the surface of the mistaken notion respecting the direction in which object is perfectly plain and smooth. If MN, the object was illuminated. fig. 15, is the surface of a mahogany table, MN 307. The last species of illusion of this nature, nm a section of it, and abc a section of a knot and perhaps the most remarkable of all of thein, in the wood, then it often happens, from the may be produced by a continued effort of the transparency of the thin edge at a, next the can mind to deceive itself. If we take one of the indle, that that side is illuminated while the oppo- taglio moulds used for making bas-reliefs, and site side at c is dark, the eye being placed in the direct the eye steadily to it without noticing surplare of the section abc. The consequence of rounding objects, we may entice ourselves into this is that the spot abc appears to be a hollow the belief that the intaglio is actually a bas-relief. in the table.
It is difficult at first to produce the deception, 303. Hence arises the appearance in certain but a little practice never fails to accomplish it. plates of agate, which has obtained for it the Dr. Brewster states that he has succeeded in car name of hammered agate. The surface on which rying this deception so far as to be able, by the these cavities appears is a section of small sphe eye alone, to raise a complete hollow mask of the rical aggregations of siliceous matter like abc in human face into a projecting head. In order to fig. 15, which present exactly the same phenome- do this we must exclude the vision of other obnon, arising from the same cause as the knots in jects: and also the margin or thickness of the cast. mahogany and other woods.
308. The phenomena arising from atmospheric 304. The very same phenomenon is, as we refraction may be best understood by an examinahave already stated, often seen in mother-of-pearl. tion of the apparent alteration in the colors of Indeed it is so common in this substance that it the heavens is almost impossible to find a mother-of-pearl 309. If the light of the setting-sun, by pass. counter which seems to have its surface flat, al- ing through a long tract of air, be divested of a though they are perfectly so when examined by portion of its rays, the remainder, which is transthe touch. Owing to the refraction of the light mitted, will illuminate the western clouds with by the different growths of the shell lying in dif- an orange color, and as the sun sets more and ferent planes, the flattest surface seems to be un- more, a great number are reflected, while the equal and undulating.
clouds grow more deeply red, till at length the 305. Dr. Brewster states that one of the finest entire disappearance of the sun leaves them of a deceptions which he ever met with, arising from leaden hue. the disposition of light and shadow, presented 310. When a direct spectrum is thrown on itself on viewing through a telescope the surface colors darker than itself, it mixes with them; as of a growing field of corn, illuminated by the the yellow spectrum of the setting-sun, thrown sun when near the horizon. This field, on Sir on the green grass, becomes a greener yellow. Walter Scott's estate at Abbotsford, was about But, when a direct spectrum is thrown on colors two miles distant, and was divided into furrows, brighter than itself, it becomes instantly changed which were directed to the eye of the observer, into the reverse spectrum, which mixes with as shown in fig. 16, where A B C D E F, re- those brighter colors. So the yellow spectrum present the furrows. These furrows are of course of the setting sun thrown on the luminous sky depressed, and the growing corn rises gradually becomes blue, and changes with the color or from two adjacent ones towards the middle mn, brightness of the clouds on which it appears. op, so that the surfaces A m C, Co E, were con But the reverse spectrum mixes with every kind
The drills of corn on the highest summits of color on which it is thrown, whether brighter mn, op, caught the rays of the setting sun, which than itself or not : thus, the reverse spectrum shone
upon them very obliquely in the direction obtained by viewing a piece of yellow silk, when S s, and illuminated their summits laterally, thrown on white paper, was a lucid blue-green ; while the furrows AB, CD, EF, were in sha- when thrown on black Turkey leather became dow. The consequence of this disposition of a deep violet: and the spectrum of blue silk, the light and shade was, that the whole field thrown on white paper, was light yellow ; on seemed to be trenched, and the corn to be grow- black silk was an obscure orange; and the blue ing in the trenches as well as upon the elevated spectrum obtained from orange-colored silk, beds between them. The half furrow A B nm, thrown on yellow, became a green. being shaded on its edge A B, and illuminated 311. Of the natural phenomena produced ocon its edge min, became the elevated part of the casionally by the separation of the primary trenched ground, while the other half m n C Dap. colors the rainbow is one of the most beautiful. peared the sunk part, in consequence of the side This meteor, which in poetical language is called mn being illuminated, and its other side CD in the iris, never makes its appearance except when shade. At a certain period of the day this de- the spectator is situated between the sun and a ception did not take place, and it was dispelled shower of rain; and that this conclusion is just the moment the sun had set. The telescope had any one may satisfy himself by the following no effect whatever in producing it, as it showed experiment:-Fill a ollow glass globe with objects erect.
water, and sustend it in the sun, in such a
manner that it may be raised or lowered at 1, is refracted, and falls upon the back of the pleasure; at a certain height above the eye of drop at s; from the transparency of the drop a the spectator, who looks at it with his back to portion of it passes through towards w, but the the sun, the globe will appear to be red; let it remainder of it is reflected towards' t; here then be slowly lowered, and it will appear to be again, for the same reason as before, part of it orange, and afterwards, in succession, as it emerges from the drop, in the direction r, but descends, it will appear yellow, green, blue, the portion still left is reflected to u, where it is indigo, and violet. Hence the same drop of refracted towards the spectator, with the red rays rain, which must be considered as a little globe, uppermost. The great quantity of light lost at supplies all the seven colors to the eye. There each reflection is the cause of the indistinctness are sometimes two rainbows seen at the same of this bow, and therefore we cannot be surtime, one within the other, and, what may seem prised that we rarely, if ever, see bows formed remarkable, the order of the colors of the exte- by a still greater number of reflections within rior bow is the reverse of that of the interior the drops; for, though they may exist, they are one. When two bows are seen, the exterior one too faint to be seen. The secondary bow canis comparatively faint, and it is, therefore, some not be seen when the elevation of the sun is times called the false or secondary bow; while above 54° 7', and it is broader than the interior the greater distinctness of the interior one has bow, because the rays are more dispersed before obtained for it the appellation of the primary they reach the eye. bow. The rainbow was one of those phenome 315. The marine or sea bow is a phenomenon na which astonished and perplexed the ancients; sometimes observed in a much agitated sea; and, after many absurd and unsuceessful conjec- when the wind, sweeping part of the tops of the tures, their best philosophers, Pliny and Plu- waves, carries them aloft, so that the sun's rays tarch, relinquished the enquiry as one which was falling upon them, are refracted, &c., as in a above the reach of human investigation. In the common shower, and paint the colors of the bow. year 1611 Antonio de Dominis made a consi 316. Rohault mentions colored bows on the derable advance, however, towards the theory of grass, formed by the refraction of the sun's rays the rainbow, by suspènding a glass globe in the in the morning dew. Dr. Langwith, indeed, sun's light, when he found that, while he stood once saw a bow lying on the ground, the colors with his back to the sun, the colors of the rain- of which were almost as lively as those of the bow were reflected to his eye in succession by common rainbow. It was extended several hunthe globe, as it was moved higher or lower. He dred yards, and was not round, but oblong, was, however, unable to account for the produc- being as he conceived the portion of an hypertion of the different colors, as the experiments bola. The colors took up less space, and were with the prism had not yet been made, and it much more lively in those parts of the bow was reserved for Newton to perfect the discovery. which were near him than in those which were
312. To trace the progress of a ray of light at a distance. through a drop of rain in each of these bows, 317. We may now notice a peculiar perspectwill explain the cause of their differing in bright- ive appearance of aërial light and shade. It ness. In the true or primary bow, the rays of occurred on the evening of August 19th, 1826 ; light arrive at the spectator's eye, after two re- and was observed from the undercliff at the fractions and one retlection.
back of the Isle of Wight, just above Puckaster 313. Thus, let A, in plate V. fig. 1, be a drop Cove. The sky was clear; the sun had just set of rain, and S a ray from the sun falling on the to those who were standing where the appearupper part of the drop. It will suffer a refrac- ance was observed; when several enormous rays tion, and, instead of going forward in a right of light and shade were remarked towards the line, it will be bent to n; at n part of it will east, north-east, and south-east, all radiating in energe, but the remainder will be reflected to g; straight lines from a spot rather south of tast, at g it will be again refracted on passing into and just upon the horizon. They were ten or the air towards the eye at h; being thus twice twelve in number, did not join at the place refracted and once reflected, the ray is separated whence they appeared to originate, but seemed into its primitive colors ; the red part, which is to emerge from an obscure portion of surface of least thrown out of its course, makes an angle, a convex form, 8° or go in horizontal extent, and at its emergence, with the incident solar ray of about the third of that in height. The rays 40° 2', as S fh; and the violet, being the most extended from 30° to 40° on the right and left easily thrown out of its course, makes with the from the centre, but were of less extent as they solar light an angle of 40° 17'. The different became more vertical. They diminished graducolors, therefore, at the distance of the specta- ally in intensity at the extremities, until they tor, are considerably separated, and affect the could be traced no farther. The appearance eye in succession with the seven colors; but the slowly faded away, some of the rays disappearsuccession is so quick, and so many drops falling before others, but was observed upon the through the same circuit in the same time, that whole for about half an hour. the mind loses the idea of succession, and the 318. At first the phenomenon seemed inexbow seems permanent as long as the shower plicable, but after a little consideration continues in a proper direction for the eye. referred (and as it appeared from after observa
314. The exterior or secondary bow is formed tions correctly) to an effect of aerial perspective. by two reflections and two refractions. Let B The rays which seemed to originate from a comrepresent one of the drops of rain forming this mon centre on the east were really only the in
T, from the sun, falling upon it at tervals between long shadows caused by the
bow; a ray,