Chapter 4

[1]That is (according to the author's statement 150. 151.) both tend to red; the yellow deepening to orange as the comparatively dark medium is thickened before brightness; the blue deepening to violet as the light medium is thinned before darkness.—T.

178.

Dioptrical colours of both classes are closely connected, as will presently appear on a little examination. Those of the first class appeared through semi-transparent mediums, those of the second class will now appear through transparent mediums. But since every substance, however transparent, may be already considered to partake of the opposite quality (as every accumulation of a medium called transparent proves), so the near affinity of the two classes is sufficiently manifest.

179.

We will, however, first consider transparent mediums abstractedly as such, as entirely free from any degree of opacity, and direct our whole attention to a phenomenon which here presents itself, and which is known by the name of refraction.

180.

In treating of the physiological colours, we have already had occasion to vindicate whatwere formerly called illusions of sight, as the active energies of the healthy and duly efficient eye (2), and we are now again invited to consider similar instances confirming the constancy of the laws of vision.

181.

Throughout nature, as presented to the senses, everything depends on the relation which things bear to each other, but especially on the relation which man, the most important of these, bears to the rest. Hence the world divides itself into two parts, and the human being assubject, stands opposed to theobject. Thus the practical man exhausts himself in the accumulation of facts, the thinker in speculation; each being called upon to sustain a conflict which admits of no peace and no decision.

182.

But still the main point always is, whether the relations are truly seen. As our senses, if healthy, are the surest witnesses of external relations, so we may be convinced that, in all instances where they appear to contradict reality, they lay the greater and surer stress on true relations. Thus a distant object appears to us smaller; and precisely by this means we are aware of distance. We produced coloured appearances on colourless objects, through colourless mediums, and at the same moment ourattention was called to the degree of opacity in the medium.

183.

Thus the different degrees of opacity in so-called transparent mediums, nay, even other physical and chemical properties belonging to them, are known to our vision by means of refraction, and invite us to make further trials in order to penetrate more completely by physical and chemical means into those secrets which are already opened to our view on one side.

184.

Objects seen through mediums more or less transparent do not appear to us in the place which they should occupy according to the laws of perspective. On this fact the dioptrical colours of the second class depend.

185.

Those laws of vision which admit of being expressed in mathematical formulæ are based on the principle that, as light proceeds in straight lines, it must be possible to draw a straight line from the eye to any given object in order that it be seen. If, therefore, a case arises in which the light arrives to us in a bent or broken line, that we see the object by means of a bent or broken line, we are at once informed that themedium between the eye and the object is denser, or that it has assumed this or that foreign nature.

186.

This deviation from the law of right-lined vision is known by the general term of refraction; and, although we may take it for granted that our readers are sufficiently acquainted with its effects, yet we will here once more briefly exhibit it in its objective and subjective point of view.

187.

Let the sun shine diagonally into an empty cubical vessel, so that the opposite side be illumined, but not the bottom: let water be then poured into this vessel, and the direction of the light will be immediately altered; for a part of the bottom is shone upon. At the point where the light enters the thicker medium it deviates from its rectilinear direction, and appears broken: hence the phenomenon is called the breaking (brechung) or refraction. Thus much of the objective experiment.

188.

We arrive at the subjective fact in the following mode:—Let the eye be substituted for the sun: let the sight be directed in like mannerdiagonally over one side, so that the opposite inner side be entirely seen, while no part of the bottom is visible. On pouring in water the eye will perceive a part of the bottom; and this takes place without our being aware that we do not see in a straight line; for the bottom appears to us raised, and hence we give the term elevation (hebung) to the subjective phenomenon. Some points, which are particularly remarkable with reference to this, will be adverted to hereafter.

189.

Were we now to express this phenomenon generally, we might here repeat, in conformity with the view lately taken, that the relation of the objects is changed or deranged.

190.

But as it is our intention at present to separate the objective from the subjective appearances, we first express the phenomenon in a subjective form, and say,—a derangement or displacement of the object seen, or to be seen, takes place.

191.

But that which is seen without a limiting outline may be thus affected without our perceiving the change. On the other hand, if what we look at has a visible termination, we have an evident indication that a displacement occurs. If, therefore,we wish to ascertain the relation or degree of such a displacement, we must chiefly confine ourselves to the alteration of surfaces with visible boundaries; in other words, to the displacement of circumscribed objects.

192.

The general effect may take place through parallel mediums, for every parallel medium displaces the object by bringing it perpendicularly towards the eye. The apparent change of position is, however, more observable through mediums that are not parallel.

193.

These latter may be perfectly spherical, or may be employed in the form of convex or concave lenses. We shall make use of all these as occasion may require in our experiments. But as they not only displace the object from its position, but alter it in various ways, we shall, in most cases, prefer employing mediums with surfaces, not, indeed, parallel with reference to each other, but still altogether plane, namely, prisms. These have a triangle for their base, and may, it is true, be considered as portions of a lens, but they are particularly available for our experiments, inasmuch as they very perceptibly displace the object from its position, without producing a remarkable distortion.

194.

And now, in order to conduct our observations with as much exactness as possible, and to avoid all confusion and ambiguity, we confine ourselves at first to

in which, namely, the object is seen by the observer through a refracting medium. As soon as we have treated these in due series, the objective experiments will follow in similar order.

195.

Refraction can visibly take place without our perceiving an appearance of colour. To whatever extent a colourless or uniformly coloured surface may be altered as to its position by refraction, no colour consequent upon refraction appears within it, provided it has no outline or boundary. We may convince ourselves of this in various ways.

196.

Place a glass cube on any larger surface, and look through the glass perpendicularly or obliquely, the unbroken surface opposite the eye appears altogether raised, but no colour exhibits itself. If we look at a pure grey or blue sky or a uniformly white or coloured wall through a prism, the portion of the surface which the eye thus embraces will be altogether changed as to its position, without our therefore observing the smallest appearance of colour.

197.

Although in the foregoing experiments we have found all unbroken surfaces, large or small, colourless, yet at the outlines or boundaries, where the surface is relieved upon a darker or lighter object, we observe a coloured appearance.

198.

Outline, as well as surface, is necessary to constitute a figure or circumscribed object. We therefore express the leading fact thus: circumscribed objects must be displaced by refraction in order to the exhibition of an appearance of colour.

199.

We place before us the simplest object, a light disk on a dark ground (A).[1]A displacement occurs with regard to this object, if we apparently extend its outline from the centre by magnifying it. This may be done with any convex glass, and in this case we see a blue edge (B).

200.

We can, to appearance, contract the circumference of the same light disk towards the centre by diminishing the object; the edge will then appear yellow (C). This may be done with a concave glass, which, however, should not be ground thin like common eye-glasses, but must have some substance. In order, however, to make this experiment at once with the convex glass, let a smaller black disk be inserted within the light disk on a black ground. If we magnify the black disk on a white ground with a convex glass, the same result takes place as if we diminished the white disk; for we extend the black outline upon the white, and we thus perceive the yellow edge together with the blue edge (D).

201.

These two appearances, the blue and yellow, exhibit themselves in and upon the white: theyboth assume a reddish hue, in proportion as they mingle with the black.[2]

Plate 2.

202.

In this short statement we have described the primordial phenomena of all appearance of colour occasioned by refraction. These undoubtedly may be repeated, varied, and rendered more striking; may be combined, complicated, confused; but, after all, may be still restored to their original simplicity.

203.

In examining the process of the experiment just given, we find that in the one case we have, to appearance, extended the white edge upon the dark surface; in the other we have extended the dark edge upon the white surface, supplanting one by the other, pushing one over the other. We will now endeavour, step by step, to analyse these and similar cases.

204.

If we cause the white disk to move, in appearance, entirely from its place, which can be doneeffectually by prisms, it will be coloured according to the direction in which it apparently moves, in conformity with the above laws. If we look at the diska[3]through a prism, so that it appear moved tob, the outer edge will appear blue and blue-red, according to the law of the figure B (fig. 1), the other edge being yellow, and yellow-red, according to the law of the figure C (fig. 1). For in the first case the white figure is, as it were, extended over the dark boundary, and in the other case the dark boundary is passed over the white figure. The same happens if the disk is, to appearance, moved fromatoc, fromatod, and so throughout the circle.

205.

As it is with the simple effect, so it is with more complicated appearances. If we look through a horizontal prism (a b[4]) at a white disk placed at some distance behind it ate, the disk will be raised tof, and coloured according to the above law. If we remove this prism, and look through a vertical one (c d) at the same disk, it will appear ath, and coloured according to the same law. If we place the two prisms one upon the other, the disk will appear displaced diagonally, in conformity with a general law of nature, and will be coloured asbefore; that is, according to its movement in the direction,e.g.:[5]

206.

If we attentively examine these opposite coloured edges, we find that they only appear in the direction of the apparent change of place. A round figure leaves us in some degree uncertain as to this: a quadrangular figure removes all doubt.

207.

The quadrangular figurea,[6]moved in the directiona bora dexhibits no colour on the sides which are parallel with the direction in which it moves: on the other hand, if moved in the directiona c, parallel with its diagonal, all the edges of the figure appear coloured.[7]

208.

Thus, a former position (203) is here confirmed; viz. to produce colour, an object must be so displaced that the light edges be apparently carried over a dark surface, the dark edges over a light surface, the figure over its boundary, the boundary over the figure. Butif the rectilinear boundaries of a figure could be indefinitely extended by refraction, so that figure and background might only pursue their course next, but not over each other, no colour would appear, not even if they were prolonged to infinity.

[1]Plate 2, fig. 1.

[2]The author has omitted the orange and purple in the coloured diagrams which illustrate these first experiments, from a wish probably to present the elementary contrast, on which he lays a stress, in greater simplicity. The reddish tinge would be apparent, as stated above, where the blue and yellow are in contact with the black.—T.

[3]Plate 2, fig. 2

[4]Plate 2, fig. 4

[5]In this case, according to the author, the refracting medium being increased in mass, the appearance of colour is increased, and the displacement is greater.—T.

[6]Plate 2, fig. 3.

[7]Fig. 2,plate 1, contains a variety of forms, which, when viewed through a prism, are intended to illustrate the statement in this and the following paragraph.

209.

We have seen in the foregoing experiments that all appearance of colour occasioned by refraction depends on the condition that the boundary or edge be moved in upon the object itself, or the object itself over the ground, that the figure should be, as it were, carried over itself, or over the ground. And we shall now find that, by increased displacement of the object, the appearance of colour exhibits itself in a greater degree. This takes place in subjective experiments, to which, for the present, we confine ourselves, under the following conditions.

210.

First, if, in looking through parallel mediums, the eye is directed more obliquely.

Secondly, if the surfaces of the medium are no longer parallel, but form a more or less acute angle.

Thirdly, owing to the increased proportion of the medium, whether parallel mediums be increased in size, or whether the angle be increased, provided it does not attain a right angle.

Fourthly, owing to the distance of the eye armed with a refracting medium from the object to be displaced.

Fifthly, owing to a chemical property that may be communicated to the glass, and which may be afterwards increased in effect.

211.

The greatest change of place, short of considerable distortion of the object, is produced by means of prisms, and this is the reason why the appearance of colour can be exhibited most powerfully through glasses of this form. Yet we will not, in employing them, suffer ourselves to be dazzled by the splendid appearances they exhibit, but keep the above well-established, simple principles calmly in view.

212.

The colour which is outside, or foremost, in the apparent change of an object by refraction, is always the broader, and we will henceforth call this aborder: the colour that remains next the outline is the narrower, and this we will call anedge.

213.

If we move a dark boundary towards a light surface, the yellow broader border is foremost, and the narrower yellow-red edge follows close to the outline. If we move a light boundary towards a dark surface, the broader violet border is foremost, and the narrower blue edge follows.

214.

If the object is large, its centre remains uncoloured. Its inner surface is then to be considered as unlimited (195): it is displaced, but not otherwise altered: but if the object is so narrow, that under the above conditions the yellow border can reach the blue edge, the space between the outlines will be entirely covered with colour. If we make this experiment with a white stripe on a black ground,[1]the two extremes will presently meet, and thus produce green. We shall then see the following series of colours:—

Yellow-red.Yellow.Green.Blue.Blue-red.

215.

If we place a black band, or stripe, on white paper,[2]the violet border will spread till it meetsthe yellow-red edge. In this case the intermediate black is effaced (as the intermediate white was in the last experiment), and in its stead a splendid pure red will appear.[3]The series of colours will now be as follows:—

Blue.Blue-red.Red.Yellow-red.Yellow.

216.

The yellow and blue, in the first case (214), can by degrees meet so fully, that the two colours blend entirely in green, and the order will then be,

Yellow-red.Green.Blue-red.

In the second case (215), under similar circumstances, we see only

Blue.Red.Yellow.

This appearance is best exhibited by refracting the bars of a window when they are relieved on a grey sky.[4]

217.

In all this we are never to forget that this appearance is not to be considered as a complete or final state, but always as a progressive, increasing, and, in many senses, controllable appearance. Thus we find that, by the negation of the above five conditions, it gradually decreases, and at last disappears altogether.

[1]Plate 2, fig. 5,left.

[2]Plate 2, fig. 5,right.

[3]This pure red, the union of orange and violet, is considered by the author the maximum of the coloured appearance: he has appropriated the termpurpurto it. See paragraph703, andnote.—T.

[4]The bands or stripes in fig. 4,plate 1, when viewed through a prism, exhibit the colours represented inplate 2, fig. 5.

218.

Before we proceed further, it is incumbent on us to explain the first tolerably simple phenomenon, and to show its connexion with the principles first laid down, in order that the observer of nature may be enabled clearly to comprehend the more complicated appearances that follow.

219.

In the first place, it is necessary to remember that we have to do with circumscribed objects. In the act of seeing, generally, it is the circumscribed visible which chiefly invites our observation; and in the present instance, in speaking of the appearance of colour, as occasioned by refraction, the circumscribed visible, the detached object solely occupies our attention.

220.

For our chromatic exhibitions we can, however, divide objects generally intoprimaryandsecondary. The expressions of themselves denote what we understand by them, but our meaning will be rendered still more plain by what follows.

221.

Primary objects may be considered firstly asoriginal, as images which are impressed on the eye by things before it, and which assure us of their reality. To these the secondary images may be opposed asderivedimages, which remain in the organ when the object itself is taken away; those apparent after-images, which have been circumstantially treated of in the doctrine of physiological colours.

222.

The primary images, again, may be considered asdirectimages, which, like the original impressions, are conveyed immediately from the object to the eye. In contradistinction to these, the secondary images may be considered asindirect, being only conveyed to us, as it were, at second-hand from a reflecting surface. These are the mirrored, or catoptrical, images, which in certain cases can also become double images:

223.

When, namely, the reflecting body is transparent,and has two parallel surfaces, one behind the other: in such a case, an image may be reflected to the eye from both surfaces, and thus arise double images, inasmuch as the upper image does not quite cover the under one: this may take place in various ways.

Let a playing-card be held before a mirror. We shall at first see the distinct image of the card, but the edge of the whole card, as well as that of every spot upon it, will be bounded on one side with a border, which is the beginning of the second reflection. This effect varies in different mirrors, according to the different thickness of the glass, and the accidents of polishing. If a person wearing a white waistcoat, with the remaining part of his dress dark, stands before certain mirrors, the border appears very distinctly, and in like manner the metal buttons on dark cloth exhibit the double reflection very evidently.

224.

The reader who has made himself acquainted with our former descriptions of experiments (80) will the more readily follow the present statement. The window-bars reflected by plates of glass appear double, and by increased thickness of the glass, and a due adaptation of the angle of reflection, the two reflections may be entirely separated from each other. So a vase full ofwater, with a plane mirror-like bottom, reflects any object twice, the two reflections being more or less separated under the same conditions. In these cases it is to be observed that, where the two reflections cover each other, the perfect vivid image is reflected, but where they are separated they exhibit only weak, transparent, and shadowy images.

225.

If we wish to know which is the under and which the upper image, we have only to take a coloured medium, for then a light object reflected from the under surface is of the colour of the medium, while that reflected from the upper surface presents the complemental colour. With dark objects it is the reverse; hence black and white surfaces may be here also conveniently employed. How easily the double images assume and evoke colours will here again be striking.

226.

Thirdly, the primary images may be considered asprincipalimages, while the secondary can be, as it were, annexed to these asaccessoryimages. Such an accessory image produces a sort of double form; except that it does not separate itself from the principal object, although it may be said to be always endeavouring to do so. It iswith secondary images of this last description that we have to do in prismatic appearances.

227.

A surface without a boundary exhibits no appearance of colour when refracted (195). Whatever is seen must be circumscribed by an outline to produce this effect. In other words a figure, an object, is required; this object undergoes an apparent change of place by refraction: the change is however not complete, not clean, not sharp; but incomplete, inasmuch as an accessory image only is produced.

228.

In examining every appearance of nature, but especially in examining an important and striking one, we should not remain in one spot, we should not confine ourselves to the insulated fact, nor dwell on it exclusively, but look round through all nature to see where something similar, something that has affinity to it, appears: for it is only by combining analogies that we gradually arrive at a whole which speaks for itself, and requires no further explanation.

229.

Thus we here call to mind that in certain cases refraction unquestionably produces double images, as is the case in Iceland spar: similardouble images are also apparent in cases of refraction through large rock crystals, and in other instances; phenomena which have not hitherto been sufficiently observed.[1]

230.

But since in the case under consideration (227) the question relates not to double but to accessory images, we refer to a phenomenon already adverted to, but not yet thoroughly investigated. We allude to an earlier experiment, in which it appeared that a sort of conflict took place in regard to the retina between a light object and its dark ground, and between a dark object and its light ground (16). The light object in this case appeared larger, the dark one smaller.

231.

By a more exact observation of this phenomenon we may remark that the forms are not sharply distinguished from the ground, but that they appear with a kind of grey, in some degree, coloured edge; in short, with an accessory image. If, then, objects seen only with the naked eye produce such effects, what may not take place when a dense medium is interposed? It is not that alone which presents itself to usin obvious operation which produces and suffers effects, but likewise all principles that have a mutual relation only of some sort are efficient accordingly, and indeed often in a very high degree.

232.

Thus when refraction produces its effect on an object there appears an accessory image next the object itself: the real form thus refracted seems even to linger behind, as if resisting the change of place; but the accessory image seems to advance, and extends itself more or less in the mode already shown (212-216).

233.

We also remarked (224) that in double images the fainter appear only half substantial, having a kind of transparent, evanescent character, just as the fainter shades of double shadows must always appear as half-shadows. These latter assume colours easily, and produce them readily (69), the former also (80); and the same takes place in the instance of accessory images, which, it is true, do not altogether quit the real object, but still advance or extend from it as half-substantial images, and hence can appear coloured so quickly and so powerfully.

234.

That the prismatic appearance is in fact anaccessory image we may convince ourselves in more than one mode. It corresponds exactly with the form of the object itself. Whether the object be bounded by a straight line or a curve, indented or waving, the form of the accessory image corresponds throughout exactly with the form of the object.[2]

235.

Again, not only the form but other qualities of the object are communicated to the accessory image. If the object is sharply relieved from its ground, like white on black, the coloured accessory image in like manner appears in its greatest force. It is vivid, distinct, and powerful; but it is most especially powerful when a luminous object is shown on a dark ground, which may be contrived in various ways.

236.

But if the object is but faintly distinguished from the ground, like grey objects on black or white, or even on each other, the accessory image is also faint, and, when the original difference of tint or force is slight, becomes hardly discernible.

237.

The appearances which are observable when coloured objects are relieved on light, dark, or coloured grounds are, moreover, well worthy of attention. In this case a union takes place between the apparent colour of the accessory image and the real colour of the object; a compound colour is the result, which is either assisted and enhanced by the accordance, or neutralised by the opposition of its ingredients.

238.

But the common and general characteristic both of the double and accessory image is semi-transparence. The tendency of a transparent medium to become only half transparent, or merely light-transmitting, has been before adverted to (147,148). Let the reader assume that he sees within or through such a medium a visionary image, and he will at once pronounce this latter to be a semi-transparent image.

239.

Thus the colours produced by refraction may be fitly explained by the doctrine of the semi-transparent mediums. For where dark passes over light, as the border of the semi-transparent accessory image advances, yellow appears; and, on the other hand, where a light outline passes over the dark background, blue appears (150,151).

240.

The advancing foremost colour is always the broader. Thus the yellow spreads over the light with a broad border, but the yellow-red appears as a narrower stripe and is next the dark, according to the doctrine of augmentation, as an effect of shade.[3]

241.

On the opposite side the condensed blue is next the edge, while the advancing border, spreading as a thinner veil over the black, produces the violet colour, precisely on the principles before explained in treating of semi-transparent mediums, principles which will hereafter be found equally efficient in many other cases.

242.

Since an analysis like the present requires to be confirmed by ocular demonstration, we beg every reader to make himself acquainted with the experiments hitherto adduced, not in a superficial manner, but fairly and thoroughly. We have not placed arbitrary signs before him instead of the appearances themselves; no modes of expression are here proposed for hisadoption which may be repeated for ever without the exercise of thought and without leading any one to think; but we invite him to examine intelligible appearances, which must be present to the eye and mind, in order to enable him clearly to trace these appearances to their origin, and to explain them to himself and to others.

[1]The date of the publication, 1810, is sometimes to be remembered.—T.

[2]The forms in fig. 2,plate 1, when seen through a prism, are again intended to exemplify this. In the plates to the original work curvilinear figures are added, but the circles, fig. 1, in the same plate, may answer the same end.—T.

[3]The author has before observed that colour is a degree of darkness, and he here means that increase of darkness, produced by transparent mediums, is, to a certain extent, increase of colour.—T.

243.

We need only take the five conditions (210) under which the appearance of colour increases in the contrary order, to produce the contrary or decreasing state; it may be as well, however, briefly to describe and review the corresponding modifications which are presented to the eye.

244.

At the highest point of complete junction of the opposite edges, the colours appear as follows (216):—

245.

Where the junction is less complete, the appearance is as follows (214,215):—

Here, therefore, the surface still appears completely coloured, but neither series is to be considered as an elementary series, always developing itself in the same manner and in the same degrees; on the contrary, they can and should be resolved into their elements; and, in doing this, we become better acquainted with their nature and character.

246.

These elements then are (199,200,201)—

Here the surface itself, the original object, which has been hitherto completely covered, and as it were lost, again appears in the centre of the colours, asserts its right, and enables usfully to recognise the secondary nature of the accessory images which exhibit themselves as "edges" and "borders."—Note N.

247.

We can make these edges and borders as narrow as we please; nay, we can still have refraction in reserve after having done away with all appearance of colour at the boundary of the object.

Having now sufficiently investigated the exhibition of colour in this phenomenon, we repeat that we cannot admit it to be an elementary phenomenon. On the contrary, we have traced it to an antecedent and a simpler one; we have derived it, in connexion with the theory of secondary images, from the primordial phenomenon of light and darkness, as affected or acted upon by semi-transparent mediums. Thus prepared, we proceed to describe the appearances which refraction produces on grey and coloured objects, and this will complete the section of subjective phenomena.

248.

Hitherto we have confined our attention to black and white objects relieved on respectively opposite grounds, as seen through the prism, because the coloured edges and borders are most clearly displayed in such cases. We now repeat these experiments with grey objects, and again find similar results.

249.

As we called black the equivalent of darkness, and white the representative of light (18), so we now venture to say that grey represents half-shadow, which partakes more or less of light and darkness, and thus stands between the two. We invite the reader to call to mind the following facts as bearing on our present view.

250.

Grey objects appear lighter on a black than on a white ground (33); they appear as a light on a black ground, and larger; as a dark on the white ground, and smaller. (16.)

251.

The darker the grey the more it appears as a faint light on black, as a strong dark on white, andvice versâ; hence the accessory images of dark-grey on black are faint, on white strong: so the accessory images of light-grey on white are faint, on black strong.

252.

Grey on black, seen through the prism, will exhibit the same appearances as white on black; the edges are coloured according to the same law, only the borders appear fainter. If we relieve grey on white, we have the same edges and borders which would be produced if we saw black on white through the prism.—Note O.

253.

Various shades of grey placed next each other in gradation will exhibit at their edges, either blue and violet only, or red and yellow only, according as the darker grey is placed over or under.

254.

A series of such shades of grey placed horizontally next each other will be coloured conformably to the same law according as the whole series is relieved, on a black or white ground above or below.

255.

The observer may see the phenomena exhibited by the prism at one glance, by enlarging the plate intended to illustrate this section.[1]

256.

It is of great importance duly to examine and consider another experiment in which a grey object is placed partly on a black and partly on a white surface, so that the line of division passes vertically through the object.

257.

The colours will appear on this grey object in conformity with the usual law, but according to the opposite relation of the light to the dark, and will be contrasted in a line. For as the grey is as a light to the black, so it exhibits the red and yellow above the blue and violet below: again, as the grey is as a dark to the white, the blue and violet appear above the red and yellow below. This experiment will be found of great importance with reference to the next chapter.

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