Works of Johann Wolfgang von Goethe

by Johann Wolfgang von Goethe

  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: similar double images are also apparent in cases of refraction through large rock crystals, and in other instances; phenomena which have not hitherto been sufficiently observed.

  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 us in 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 an accessory 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.

  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 semitransparent 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.

  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.

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  XVI. Decrease of the Appearance of Colour.

  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): —

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

  245

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

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

  Here, therfore, 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) —

  Yellow-red. Blue. Yellow. Blue-red. White. Black. Blue. Yellow-red. Blue-red. Yellow.

  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 us fully to recognise the secondary nature of the accessory images which exhibit themselves as “edges” and “borders.” — Note N.

  247

  We c
an 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.

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  XVII. Grey Objects Displaced By Refraction.

  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, and vice versa; 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.

  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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  XVIII. Coloured Objects Displaced By Refraction.

  258

  An unlimited coloured surface exhibits no prismatic colour in addition to its own hue, thus not at all differing from a black, white, or grey surface. To produce the appearance of colour, light and dark boundaries must act on it either accidentally or by contrivance. Hence experiments and observations on coloured surfaces, as seen through the prism, can only be made when such surfaces are separated by an outline from another differently tinted surface, in short when circumscribed objects are coloured,

  259

  All colours, whatever they may be, correspond so far with grey, that they appear darker than white and lighter than black. This shade-like quality of colour has been already alluded to (69), and will become more and more evident. If then we begin by placing coloured objects on black and white surfaces, and examine them through the prism, we shall again have all that we have seen exhibited with grey surfaces.

  260

  If we displace a coloured object by refraction, there appears, as in the case of colourless objects and according to the same laws, an accessory image. This accessory image retains, as far as colour is concerned, its usual nature, and acts on one side as a blue and blue-red, on the opposite side as a yellow and yellow-red. Hence the apparent colour of the edge and border will be either homogeneous with the real colour of the object, or not so. In the first case the apparent image identifies itself with the real one, and appears to increase it, while, in the second case, the real image may be vitiated, rendered indistinct, and reduced in size by the apparent image. We proceed to review the cases in which these effects are most strikingly exhibited.

  261

  If we take a coloured drawing enlarged from the plate, which illustrates this experiment, and examine the red and blue squares placed next each other on a black ground, through the prism as usual, we shall find that as both colours are lighter than the ground, similarly coloured edges and borders will appear above and below, at the outlines of both, only they will not appear equally distinct to the eye.

  262

  Red is proportionally much lighter on black than blue is. The colours of the edges will therefore appear stronger on the red than on the blue, which here acts as a dark-grey, but little different from black.. (251.)

  263

  The extreme red edge will identify itself with the vermilion colour of the square, which will thus appear a little elongated in this direction; while the yellow border immediately underneath it only gives the red surface a more brilliant appearance, and is not distinguished without attentive observation.

  264

  On the other hand the red edge and yellow border are heterogeneous with the blue square; a dull red appears at the edge, and a dull green mingles with the figure, and thus the blue square seems, at a hasty glance, to be comparatively diminished on this side.

  265

  At the lower outline of the two squares a blue edge and a violet border will appear, and will produce the contrary effect; for the blue edge, which is heterogeneous with the warm red surface, will vitiate it and produce a neutral colour, so that the red on this side appears comparatively reduced and driven upwards, and the violet border on the black is scarcely perceptible.

  266

  On the other hand, the blue apparent edge will identify itself with the blue square, and not only not reduce, but extend it. The blue edge and even the violet border next it have the apparent effect of increasing the surface, and elongating it in that direction.

  267

  The effect of homogeneous and heterogeneous edges, as I have now minutely described it, is so powerful and singular that the two squares at the first glance seem pushed out of their relative horizontal position and moved in opposite directions, the red upwards, the blue downwards. But no one who is accustomed to observe experiments in a certain succession, and respectively to connect and trace them, will suffer himself to be deceived by such an unreal effect.

  268

  A just impression with regard to this important phenomenon will, however, much depend on some nice and even troublesome conditions, which are necessary to produce the illusion in question. Paper should be tinged with vermilion or the best minium for the red square, and with deep indigo for the blue square. The blue and red prismatic edges will then unite imperceptibly with the real surfaces where they are respectively homogeneous; where they are not, they vitiate the colours of the squares without producing a very distinct middle tint. The real r
ed should not incline too much to yellow, otherwise the apparent deep red edge above will be too distinct; at the same time it should be somewhat yellow, otherwise the transition to the yellow border will be too observable. The blue must not be light, otherwise the red edge will be visible, and the yellow border will produce a too decided green, while the violet border underneath would not give us the impression of being part of an elongated light blue square.

  269

  All this will be treated more circumstantially hereafter, when we speak of the apparatus intended to facilitate the experiments connected with this part of our subject. Every inquirer should prepare the figures himself, in order fairly to exhibit this specimen of ocular deception, and at the same time to convince himself that the coloured edges, even in this case, cannot escape accurate examination.

  270

  Meanwhile various other combinations, as exhibited in the plate, are fully calculated to remove all doubt on this point in the mind of every attentive observer.

  271

  If, for instance, we look at a white square, next the blue one, on a black ground, the prismatic hues of the opposite edges of the white, which here occupies the place of the red in the former experiment, will exhibit themselves in their utmost force. The red edge extends itself above the level of the blue almost in a greater degree than was the case with the red square itself in the former experiment. The lower blue edge, again, is visible in its full force next the white, while, on the other hand, it cannot be distinguished next the blue square. The violet border underneath is also much more apparent on the white than on the blue.

  272

  If the observer now compares these double squares, carefully prepared and arranged one above the other, the red with the white, the two blue squares together, the blue with the red, the blue with the white, he will clearly perceive the relations of these surfaces to their coloured edges and borders.

  273

  The edges and their relations to the coloured surfaces appear still more striking if we look at the coloured squares and a black square on a white ground; for in this case the illusion before mentioned ceases altogether, and the effect of the edges is as visible as in any case that has come under our observation. Let the blue and red squares be first examined through the prism. In both the blue edge now appears above; this edge, homogeneous with the blue surface, unites with it, and appears to extend it upwards, only the blue edge, owing to its lightness, is somewhat too distinct in its upper portion; the violet border underneath it is also sufficiently evident on the blue. The apparent blue edge is, on the other hand, heterogeneous with the red square; it is neutralised by contrast, and is scarcely visible; meanwhile the violet border, uniting with the real red, produces a hue resembling that of the peach-blossom.