XXI.

REFRACTION WITHOUT THE APPEARANCE OF COLOUR.

306 (195, 196).

That refraction may exhibit its effects without producing an appearance

of colour, is not to be demonstrated so perfectly in objective as

in subjective experiments. We have, it is true, unlimited spaces

which we can look at through the prism, and thus convince ourselves

that no colour appears where there is no boundary; but we have no

unlimited source of light which we can cause to act through the prism.

Our light comes to us from circumscribed bodies; and the sun, which

chiefly produces our prismatic appearances, is itself only a small,

circumscribed, luminous object.

307.

We may, however, consider every larger opening through which the sun

shines, every larger medium through which the sun-light is transmitted

and made to deviate from its course, as so far unlimited that we can

confine our attention to the centre of the surface without considering

its boundaries.

308 (197).

If we place a large water-prism in the sun, a large bright space is

refracted upwards by it on the plane intended to receive the image, and

the middle of this illumined space will be colourless. The same effect

may be produced if we make the experiment with glass prisms having

angles of few degrees: the appearance may be produced even through

glass prisms, whose refracting angle is sixty degrees, provided we

place the recipient surface near enough.

XXII.

CONDITIONS OF THE APPEARANCE OF COLOUR.

309 (198).

Although, then, the illumined space before mentioned appears indeed

refracted and moved from its place, but not coloured, yet on the

horizontal edges of this space we observe a coloured appearance.

That here again the colour is solely owing to the displacement of a

circumscribed object may require to be more fully proved.

The luminous body which here acts is circumscribed: the sun, while it

shines and diffuses light, is still an insulated object. However small

the opening in the lid of a camera obscura be made, still the whole

image of the sun will penetrate it. The light which streams from all

parts of the sun's disk, will cross itself in the smallest opening, and

form the angle which corresponds with the sun's apparent diameter. On

the outside we have a cone narrowing to the orifice; within, this apex

spreads again, producing on an opposite surface a round image, which

still increases in size in proportion to the distance of the recipient

surface from the apex. This image, together with all other objects

of the external landscape, appears reversed on the white surface in

question in a dark room.

310.

How little therefore we have here to do with single sun-rays, bundles

or fasces of rays, cylinders of rays, pencils, or whatever else of the

kind may be imagined, is strikingly evident. For the convenience of

certain diagrams the sun-light may be assumed to arrive in parallel

lines, but it is known that this is only a fiction; a fiction quite

allowable where the difference between the assumption and the true

appearance is unimportant; but we should take care not to suffer such a

postulate to be equivalent to a fact, and proceed to further operations

on such a fictitious basis.

311.

Let the aperture in the window-shutter be now enlarged at pleasure, let

it be made round or square, nay, let the whole shutter be opened, and

let the sun shine into the room through the whole window; the space

which the sun illumines will always be larger according to the angle

which its diameter makes; and thus even the whole space illumined by

the sun through the largest window is only the image of the sun _plus_

the size of the opening. We shall hereafter have occasion to return to

this.

312 (199).

If we transmit the image of the sun through convex glasses we contract

it towards the focus. In this case, according to the laws before

explained, a yellow border and a yellow-red edge must appear when the

spectrum is thrown on white paper. But as this experiment is dazzling

and inconvenient, it may be made more agreeably with the image of the

full moon. On contracting this orb by means of a convex glass, the

coloured edge appears in the greatest splendour; for the moon transmits

a mitigated light in the first instance, and can thus the more readily

produce colour which to a certain extent accompanies the subduing of

light: at the same time the eye of the observer is only gently and

agreeably excited.

313 (200).

If we transmit a luminous image through concave glasses, it is

dilated. Here the image appears edged with blue.

314.

The two opposite appearances may be produced by a convex glass,

simultaneously or in succession; simultaneously by fastening an opaque

disk in the centre of the convex glass, and then transmitting the sun's

image. In this case the luminous image and the black disk within it are

both contracted, and, consequently, the opposite colours must appear.

Again, we can present this contrast in succession by first contracting

the luminous image towards the focus, and then suffering it to expand

again beyond the focus, when it will immediately exhibit a blue edge.

315 (201).

Here too what was observed in the subjective experiments is again to be

remarked, namely, that blue and yellow appear in and upon the white,

and that both assume a reddish appearance in proportion as they mingle

with the black.

316 (202, 203).

These elementary phenomena occur in all subsequent objective

experiments, as they constituted the groundwork of the subjective

ones. The process too which takes place is the same; a light boundary

is carried over a dark surface, a dark surface is carried over a light

boundary. The edges must advance, and as it were push over each other

in these experiments as in the former ones.

317 (204).

If we admit the sun's image through a larger or smaller opening into

the dark room, if we transmit it through a prism so placed that its

refracting angle, as usual, is underneath; the luminous image, instead

of proceeding in a straight line to the floor, is refracted upwards on

a vertical surface placed to receive it. This is the moment to take

notice of the opposite modes in which the subjective and objective

refractions of the object appear.

318.

If we _look_ through a prism, held with its refracting angle

underneath, at an object above us, the object is moved downwards;

whereas a luminous image refracted through the same prism is moved

upwards. This, which we here merely mention as a matter of fact for

the sake of brevity, is easily explained by the laws of refraction and

elevation.

319.

The luminous object being moved from its place in this manner, the

coloured borders appear in the order, and according to the laws before

explained. The violet border is always foremost, and thus in objective

cases proceeds upwards, in subjective cases downwards.

320 (205).

The observer may convince himself in like manner of the mode in which

the appearance of colour takes place in the diagonal direction when the

displacement is effected by means of two prisms, as has been plainly

enough shown in the subjective example; for this experiment, however,

prisms should be procured of few degrees, say about fifteen.

321(206, 207).

That the colouring of the image takes place here too, according to the

direction in which it moves, will be apparent if we make a _square_

opening of moderate size in a shutter, and cause the luminous image

to pass through a water-prism; the spectrum being moved first in the

horizontal and vertical directions, then diagonally, the coloured edges

will change their position accordingly.

322(208).

Whence it is again evident that to produce colour the boundaries must

be carried over each other, not merely move side by side.

XXIII.

CONDITIONS OF THE INCREASE OF COLOUR.

323 (209).

Here too an increased displacement of the object produces a greater

appearance of colour.

324 (210).

This increased displacement occurs,

1. By a more oblique direction of the impinging luminous object through

mediums with parallel surfaces.

2. By changing the parallel form for one more or less acute angled.

3. By increased proportion of the medium, whether parallel or acute

angled; partly because the object is by this means more powerfully

displaced, partly because an effect depending on the mere mass

co-operates.

4. By the distance of the recipient surface from the refracting medium

so that the coloured spectrum emerging from the prism may be said to

have a longer way to travel.

5. When a chemical property produces its effects under all these

circumstances: this we have already entered into more fully under the

head of achromatism and hyperchromatism.

325 (211).

The objective experiments have this advantage that the progressive

states of the phenomenon may be arrested and clearly represented by

diagrams, which is not the case with the subjective experiments.

326.

We can observe the luminous image after it has emerged from the prism,

step by step, and mark its increasing colour by receiving it on a

plane at different distances, thus exhibiting before our eyes various

sections of this cone, with an elliptical base: again, the phenomenon

may at once be rendered beautifully visible throughout its whole course

in the following manner:--Let a cloud of fine white dust be excited

along the line in which the image passes through the dark space; the

cloud is best produced by fine, perfectly dry, hair-powder. The more or

less coloured appearance will now be painted on the white atoms, and

presented in its whole length and breadth to the eye of the spectator.

327.

By this means we have prepared some diagrams, which will be found among

the plates. In these the appearance is exhibited from its first origin,

and by these the spectator can clearly comprehend why the luminous

image is so much more powerfully coloured through prisms than through

parallel mediums.

328 (212).

At the two opposite outlines of the image an opposite appearance

presents itself, beginning from an acute angle;[1] the appearance

spreads as it proceeds further in space, according to this angle. On

one side, in the direction in which the luminous image is moved, a

violet border advances on the dark, a narrower blue edge remains next

the outline of the image. On the opposite side a yellow border advances

into the light of the image itself, and a yellow-red edge remains at

the outline.

329 (213).

Here, therefore, the movement of the dark against the light, of the

light against the dark, may be clearly observed.

[Illustration]

330 (214).

The centre of a large object remains long uncoloured, especially with

mediums of less density and smaller angles; but at last the opposite

borders and edges touch each other, upon which a green appears in the

centre of the luminous image.

331 (215).

Objective experiments have been usually made with the sun's image: an

objective experiment with a dark object has hitherto scarcely been

thought of. We have, however, prepared a convenient contrivance for

this also. Let the large water-prism before alluded to be placed in

the sun, and let a round pasteboard disk be fastened either inside or

outside. The coloured appearance will again take place at the outline,

beginning according to the usual law; the edges will appear, they will

spread in the same proportion, and when they meet, red will appear in

the centre[2]. An intercepting square may be added near the round disk,

and placed in any direction _ad libitum_, and the spectator can again

convince himself of what has been before so often described.

332 (216).

If we take away these dark objects from the prism, in which case,

however, the glass is to be carefully cleaned, and hold a rod or a

large pencil before the centre of the horizontal prism, we shall

then accomplish the complete immixture of the violet border and the

yellow-red edge, and see only the three colours, the external blue, and

yellow, and the central red.

333.

If again we cut a long horizontal opening in the middle of a piece of

pasteboard, fastened on the prism, and then cause the sun-light to pass

through it, we shall accomplish the complete union of the yellow border

with the blue edge upon the light, and only see yellow-red, green and

violet. The details of this are further entered into in the description

of the plates.

334 (217).

The prismatic appearance is thus by no means complete and final when

the luminous image emerges from the prism. It is then only that

we perceive its elements in contrast; for as it increases these

contrasting elements unite, and are at last intimately joined. The

section of this phenomenon arrested on a plane surface is different

at every degree of distance from the prism; so that the notion of an

immutable series of colours, or of a pervading similar proportion

between them, cannot be a question for a moment.

[1] Plate 4. fig. 1.

[2] Plate 4. fig. 2.

XXIV.

EXPLANATION OF THE FOREGOING PHENOMENA.

335 (218).

As we have already entered into this analysis circumstantially while

treating of the subjective experiments, as all that was of force there

is equally valid here, it will require no long details in addition to

show that the phenomena, which are entirely parallel in the two cases,

may also be traced precisely to the same sources.

336 (219).

That in objective experiments also we have to do with circumscribed

images, has been already demonstrated at large. The sun may shine

through the smallest opening, yet the image of the whole disk

penetrates beyond. The largest prism may be placed in the open

sun-light, yet it is still the sun's image that is bounded by the

edges of the refracting surfaces, and produces the accessory images

of this boundary. We may fasten pasteboard, with many openings cut in

it, before the water-prism, yet we still merely see multiplied images

which, after having been moved from their place by refraction, exhibit

coloured edges and borders, and in these mere accessory images.

337 (235).

In subjective experiments we have seen that objects strongly relieved

from each other produce a very lively appearance of colour, and this

will be the case in objective experiments in a much more vivid and

splendid degree. The sun's image is the most powerful brightness we

know; hence its accessory image will be energetic in proportion, and

notwithstanding its really secondary dimmed and darkened character,

must be still very brilliant. The colours thrown by the sun-light

through the prism on any object, carry a powerful light with them, for

they have the highest and most intense source of light, as it were, for

their ground.

338.

That we are warranted in calling even these accessory images

semi-transparent, thus deducing the appearances from the doctrine

of the semi-transparent mediums, will be clear to every one who has

followed us thus far, but particularly to those who have supplied

themselves with the necessary apparatus, so as to be enabled at all

times to witness the precision and vivacity with which semi-transparent

mediums act.

XXV.

DECREASE OF THE APPEARANCE OF COLOUR.

339 (243).

If we could afford to be concise in the description of the decreasing

coloured appearance in subjective cases, we may here be permitted

to proceed with still greater brevity while we refer to the former

distinct statement. One circumstance, only on account of its great

importance, may be here recommended to the reader's especial attention

as a leading point of our whole thesis.

340 (244, 247).

The decline of the prismatic appearance must be preceded by its

separation, by its resolution into its elements. At a due distance from

the prism, the image of the sun being entirely coloured, the blue and

yellow at length mix completely, and we see only yellow-red, green, and

blue-red. If we bring the recipient surface nearer to the refracting

medium, yellow and blue appear again, and we see the five colours with

their gradations. At a still shorter distance the yellow and blue

separate from each other entirely, the green vanishes, and the image

itself appears, colourless, between the coloured edges and borders. The

nearer we bring the recipient surface to the prism, the narrower the

edges and borders become, till at last, when in contact with the prism,

they are reduced to nothing.

XXVI.

GREY OBJECTS.

341 (218).

We have exhibited grey objects as very important to our inquiry in the

subjective experiments. They show, by the faintness of the accessory

images, that these same images are in all cases derived from the

principal object. If we wish here, too, to carry on the objective

experiments parallel with the others, we may conveniently do this by

placing a more or less dull ground glass before the opening through

which the sun's image enters. By this means a subdued image would be

produced, which on being refracted would exhibit much duller colours on

the recipient plane than those immediately derived from the sun's disk;

and thus, even from the intense sun-image, only a faint accessory image

would appear, proportioned to the mitigation of the light by the glass.

This experiment, it is true, will only again and again confirm what is

already sufficiently familiar to us.

XXVII.

COLOURED OBJECTS.

342 (260).

There are various modes of producing coloured images in objective

experiments. In the first place, we can fix coloured glass before the

opening, by which means a coloured image is at once produced; secondly,

we can fill the water-prism with coloured fluids; thirdly, we can cause

the colours, already produced in their full vivacity by the prism, to

pass through proportionate small openings in a tin plate, and thus

prepare small circumscribed colours for a second operation. This last

mode is the most difficult; for owing to the continual progress of the

sun, the image cannot be arrested in any direction at will. The second

method has also its inconveniences, since not all coloured liquids can

be prepared perfectly bright and clear. On these accounts the first is

to be preferred, and deserves the more to be adopted because natural

philosophers have hitherto chosen to consider the colours produced

from the sun-light through the prism, those produced through liquids

and glasses, and those which are already fixed on paper or cloth, as

exhibiting effects equally to be depended on, and equally available in

demonstration.

343.

As it is thus merely necessary that the image should be coloured, so

the large water-prism before alluded to affords us the best means of

effecting this. A pasteboard screen may be contrived to slide before

the large surfaces of the prism, through which, in the first instance,

the light passes uncoloured. In this screen openings of various forms

may be cut, in order to produce different images, and consequently

different accessory images. This being done, we need only fix coloured

glasses before the openings, in order to observe what effect refraction

produces on coloured images in an objective sense.

344.

A series of glasses may be prepared in a mode similar to that before

described (284); these should be accurately contrived to slide in the

grooves of the large water-prism. Let the sun then shine through them,

and the coloured images refracted upwards will appear bordered and

edged, and will vary accordingly: for these borders and edges will be

exhibited quite distinctly on some images, and on others will be mixed

with the specific colour of the glass, which they will either enhance

or neutralize. Every observer will be enabled to convince himself

here again that we have only to do with the same simple phenomenon so

circumstantially described subjectively and objectively.

XXVIII.

ACHROMATISM AND HYPERCHROMATISM.

345 (285, 290).

It is possible to make the hyperchromatic and achromatic experiments

objectively as well as subjectively. After what has been already

stated, a short description of the method will suffice, especially as

we take it for granted that the compound prism before mentioned is in

the hands of the observer.

346.

Let the sun's image pass through an acute-angled prism of few degrees,

prepared from crown-glass, so that the spectrum be refracted upwards on

an opposite surface; the edges will appear coloured, according to the

constant law, namely, the violet and blue above and outside, the yellow

and yellow-red below and within the image. As the refracting angle of

this prism is undermost, let another proportionate prism of flint-glass

be placed against it, with its refracting angle uppermost. The sun's

image will by this means be again moved to its place, where, owing to

the excess of the colouring power of the prism of flint-glass, it will

still appear a little coloured, and, in consequence of the direction

in which it has been moved, the blue and violet will now appear

underneath and outside, the yellow and yellow-red above and inside.

347.

If the whole image be now moved a little upwards by a proportionate

prism of crown-glass, the hyperchromatism will disappear, the sun's

image will be moved from its place, and yet will appear colourless.

348.

With an achromatic object-glass composed of three glasses, this

experiment may be made step by step, if we do not mind taking out the

glasses from their setting. The two convex glasses of crown-glass in

contracting the sun's image towards the focus, the concave glass of

flint-glass in dilating the image beyond it, exhibit at the edges the

usual colours. A convex glass united with a concave one exhibits the

colours according to the law of the latter. If all three glasses are

placed together, whether we contract the sun's image towards the focus,

or suffer it to dilate beyond the focus, coloured edges never appear,

and the achromatic effect intended by the optician is, in this case,

again attained.

349.

But as the crown-glass has always a greenish tint, and as a tendency

to this hue may be more decided in large and strong object-glasses,

and under certain circumstances produce the compensatory red,

(which, however, in repeated experiments with several instruments of

this kind did not occur to us,) philosophers have resorted to the

most extraordinary modes of explaining such a result; and having

been compelled, in support of their system, theoretically to prove

the impossibility of achromatic telescopes, have felt a kind of

satisfaction in having some apparent ground for denying so great an

improvement. Of this, however, we can only treat circumstantially in

our historical account of these discoveries.

XXIX.

COMBINATION OF SUBJECTIVE AND OBJECTIVE EXPERIMENTS.

350.

Having shown above (318) that refraction, considered objectively and

subjectively, must act in opposite directions, it will follow that if

we combine the experiments, the effects will reciprocally destroy each

other.

351.

Let the sun's image be thrown upwards on a vertical plane, through

a horizontally-placed prism. If the prism is long enough to admit of

the spectator also looking through it, he will see the image elevated

by the objective refraction again depressed, and in the same place in

which it appeared without refraction.

352.

Here a remarkable case presents itself, but at the same time a natural

result of a general law. For since, as often before stated, the

objective sun's image thrown on the vertical plane is not an ultimate

or unchangeable state of the phenomenon, so in the above operation the

image is not only depressed when seen through the prism, but its edges

and borders are entirely robbed of their hues, and the spectrum is

reduced to a colourless circular form.

353.

By employing two perfectly similar prisms placed next each other, for

this experiment, we can transmit the sun's image through one, and look

through the other.

354.

If the spectator advances nearer with the prism through which he looks,

the image is again elevated, and by degrees becomes coloured according

to the law of the first prism. If he again retires till he has brought

the image to the neutralized point, and then retires still farther

away, the image, which had become round and colourless, moves still

more downwards and becomes coloured in the opposite sense, so that

if we look through the prism and upon the refracted spectrum at the

same time, we see the same image coloured according to subjective and

objective laws.

355.

The modes in which this experiment may be varied are obvious. If the

refracting angle of the prism, through which the sun's image was

objectively elevated, is greater than that of the prism through which

the observer looks, he must retire to a much greater distance, in order

to depress the coloured image so low on the vertical plane that it

shall appear colourless, and _vice versâ_.

356.

It will be easily seen that we may exhibit achromatic and

hyperchromatic effects in a similar manner, and we leave it to the

amateur to follow out such researches more fully. Other complicated

experiments in which prisms and lenses are employed together, others

again, in which objective and subjective experiments are variously

intermixed, we reserve for a future occasion, when it will be our

object to trace such effects to the simple phenomena with which we are

now sufficiently familiar.

XXX.

TRANSITION.

357.

In looking back on the description and analysis of dioptrical colours,

we do not repent either that we have treated them so circumstantially,

or that we have taken them into consideration before the other physical

colours, out of the order we ourselves laid down. Yet, before we quit

this branch of our inquiry, it may be as well to state the reasons that

have weighed with us.

358.

If some apology is necessary for having treated the theory of the

dioptrical colours, particularly those of the second class, so

diffusely, we should observe, that the exposition of any branch of

knowledge is to be considered partly with reference to the intrinsic

importance of the subject, and partly with reference to the particular

necessities of the time in which the inquiry is undertaken. In our

own case we were forced to keep both these considerations constantly

in view. In the first place we had to state a mass of experiments with

our consequent convictions; next, it was our especial aim to exhibit

certain phenomena (known, it is true, but misunderstood, and above

all, exhibited in false connection,) in that natural and progressive

development which is strictly and truly conformable to observation; in

order that hereafter, in our polemical or historical investigations,

we might be enabled to bring a complete preparatory analysis to bear

on, and elucidate, our general view. The details we have entered into

were on this account unavoidable; they may be considered as a reluctant

consequence of the occasion. Hereafter, when philosophers will look

upon a simple principle as simple, a combined effect as combined; when

they will acknowledge the first elementary, and the second complicated

states, for what they are; then, indeed, all this statement may be

abridged to a narrower form; a labour which, should we ourselves

not be able to accomplish it, we bequeath to the active interest of

contemporaries and posterity.

359.

With respect to the order of the chapters, it should be remembered

that natural phenomena, which are even allied to each other, are

not connected in any particular sequence or constant series; their

efficient causes act in a narrow circle, so that it is in some sort

indifferent what phenomenon is first or last considered; the main point

is, that all should be as far as possible present to us, in order that

we may embrace them at last from one point of view, partly according to

their nature, partly according to generally received methods.

360.

Yet, in the present particular instance, it may be asserted that the

dioptrical colours are justly placed at the head of the physical

colours; not only on account of their striking splendour and their

importance in other respects, but because, in tracing these to their

source, much was necessarily entered into which will assist our

subsequent enquiries.

361.

For, hitherto, light has been considered as a kind of abstract

principle, existing and acting independently; to a certain extent

self-modified, and on the slightest cause, producing colours out of

itself. To divert the votaries of physical science from this mode

of viewing the subject; to make them attentive to the fact, that in

prismatic and other appearances we have not to do with light as an

uncircumscribed and modifying principle, but as circumscribed and

modified; that we have to do with a luminous image; with images or

circumscribed objects generally, whether light or dark: this was the

purpose we had in view, and such is the problem to be solved.

362.

All that takes place in dioptrical cases,--especially those of the

second class which are connected with the phenomena of refraction,--is

now sufficiently familiar to us, and will serve as an introduction to

what follows.

363.

Catoptrical appearances remind us of the physiological phenomena, but

as we ascribe a more objective character to the former, we thought

ourselves justified in classing them with the physical examples. It is

of importance, however, to remember that here again it is not light, in

an abstract sense, but a luminous image that we have to consider.

364.

In proceeding onwards to the paroptrical class, the reader, if duly

acquainted with the foregoing facts, will be pleased to find himself

once more in the region of circumscribed forms. The shadows of bodies,

especially, as secondary images, so exactly accompanying the object,

will serve greatly to elucidate analogous appearances.

365.

We will not, however, anticipate these statements, but proceed as

heretofore in what we consider the regular course.

XXXI.

CATOPTRICAL COLOURS.

366.

Catoptrical colours are such as appear in consequence of a mirror-like

reflection. We assume, in the first place, that the light itself

as well as the surface from which it is reflected, is perfectly

colourless. In this sense the appearances in question come under the

head of physical colours. They arise in consequence of reflection, as

we found the dioptrical colours of the second class appear by means of

refraction. Without further general definitions, we turn our attention

at once to particular cases, and to the conditions which are essential

to the exhibition of these phenomena.

367.

If we unroll a coil of bright steel-wire, and after suffering it to

spring confusedly together again, place it at a window in the light,

we shall see the prominent parts of the circles and convolutions

illumined, but neither resplendent nor iridescent. But if the sun

shines on the wire, this light will be condensed into a point, and we

perceive a small resplendent image of the sun, which, when seen near,

exhibits no colour. On retiring a little, however, and fixing the eyes

on this refulgent appearance, we discern several small mirrored suns,

coloured in the most varied manner; and although the impression is that

green and red predominate, yet, on a more accurate inspection, we find

that the other colours are also present.

368.

If we take an eye-glass, and examine the appearance through it, we

find the colours have vanished, as well as the radiating splendour in

which they were seen, and we perceive only the small luminous points,

the repeated images of the sun. We thus find that the impression is

subjective in its nature, and that the appearance is allied to those

which we have adverted to under the name of radiating halos (100).

369.

We can, however, exhibit this phenomenon objectively. Let a piece

of white paper be fastened beneath a small aperture in the lid of a

camera-obscura, and when the sun shines through this aperture, let

the confusedly-rolled steel-wire be held in the light, so that it be

opposite to the paper. The sun-light will impinge on and in the circles

of the wire, and will not, as in the concentrating lens of the eye,

display itself in a point; but, as the paper can receive the reflection

of the light in every part of its surface will be seen in hair-like

lines, which are also iridescent.

370.

This experiment is purely catoptrical; for as we cannot imagine that

the light penetrates the surface of the steel, and thus undergoes a

change, we are soon convinced that we have here a mere reflection

which, in its subjective character, is connected with the theory of

faintly acting lights, and the after-image of dazzling lights, and as

far as it can be considered objective, announces even in the minutest

appearances, a real effect, independent of the action and reaction of

the eye.

371.

We have seen that to produce these effects not merely light but a

powerful light is necessary; that this powerful light again is not an

abstract and general quality, but a circumscribed light, a luminous

image. We can convince ourselves still further of this by analogous

cases.

372.

A polished surface of silver placed in the sun reflects a dazzling

light, but in this case no colour is seen. If, however, we slightly

scratch the surface, an iridescent appearance, in which green and red

are conspicuous, will be exhibited at a certain angle. In chased and

carved metals the effect is striking: yet it may be remarked throughout

that, in order to its appearance, some form, some alternation of light

and dark must co-operate with the reflection; thus a window-bar,

the stem of a tree, an accidentally or purposely interposed object

produces a perceptible effect. This appearance, too, may be exhibited

objectively in the camera-obscura.

373.

If we cause a polished plated surface to be so acted on by aqua fortis

that the copper within is touched, and the surface itself thus rendered

rough, and if the sun's image be then reflected from it, the splendour

will be reverberated from every minutest prominence, and the surface

will appear iridescent. So, if we hold a sheet of black unglazed paper

in the sun, and look at it attentively, it will be seen to glisten in

its minutest points with the most vivid colours.

374.

All these examples are referable to the same conditions. In the first

case the luminous image is reflected from a thin line; in the second

probably from sharp edges; in the third from very small points. In all

a very powerful and circumscribed light is requisite. For all these

appearances of colour again it is necessary that the eye should be at a

due distance from the reflecting points.

375.

If these observations are made with the microscope, the appearance

will be greatly increased in force and splendour, for we then see the

smallest portion of the surfaces, lit by the sun, glittering in these

colours of reflection, which, allied to the hues of refraction, now

attain their highest degree of brilliancy. In such cases we may observe

a vermiform iridescence on the surface of organic bodies, the further

description of which will be given hereafter.

376.

Lastly, the colours which are chiefly exhibited in reflection are red

and green, whence we may infer that the linear appearance especially

consists of a thin line of red, bounded by blue on one side and yellow

on the other. If these triple lines approach very near together, the

intermediate space must appear green; a phenomenon which will often

occur to us as we proceed.

377.

We frequently meet with these colours in nature. The colours of the

spider's web might be considered exactly of the same class with those

reflected from the steel wire, except that the non-translucent quality

of the former is not so certain as in the case of steel; on which

account some have been inclined to class the colours of the spider's

web with the phenomena of refraction.

378.

In mother-of-pearl we perceive infinitely fine organic fibres and

lamellæ in juxta-position, from which, as from the scratched silver

before alluded to, varied colours, but especially red and green, may

arise.

379.

The changing colours of the plumage of birds may also be mentioned

here, although in all organic instances a chemical principle

and an adaptation of the colour to the structure may be assumed;

considerations to which we shall return in treating of chemical colours.

380.

That the appearances of objective halos also approximate catoptrical

phenomena will be readily admitted, while we again do not deny that

refraction as well may here come into the account. For the present

we restrict ourselves to one or two observations; hereafter we may

be enabled to make a fuller application of general principles to

particular examples.

381.

We first call to mind the yellow and red circles produced on a white or

grey wall by a light placed near it (88). Light when reflected appears

subdued, and a subdued light excites the impression of yellow, and

subsequently of red.

382.

Let the wall be illumined by a candle placed quite close to it. The

farther the light is diffused the fainter it becomes; but it is still

the effect of the flame, the continuation of its action, the dilated

effect of its image. We might, therefore, very fairly call these

circles reiterated images, because they constitute the successive

boundaries of the action of the light, and yet at the same time only

present an extended image of the flame.

383.

If the sky is white and luminous round the sun owing to the atmosphere

being filled with light vapours; if mists or clouds pass before the

moon, the reflection of the disk mirrors itself in them; the halos we

then perceive are single or double, smaller or greater, sometimes very

large, often colourless, sometimes coloured.

384.

I witnessed a very beautiful halo round the moon the 15th of November,

1799, when the barometer stood high; the sky was cloudy and vapoury.

The halo was completely coloured, and the circles were concentric round

the light as in subjective halos. That this halo was objective I was

presently convinced by covering the moon's disk, when the same circles

were nevertheless perfectly visible.

385.

The different extent of the halos appears to have a relation with the

proximity or distance of the vapour from the eye of the observer.

386.

As window-panes lightly breathed upon increase the brilliancy of

subjective halos, and in some degree give them an objective character,

so, perhaps, with a simple contrivance in winter, during a quickly

freezing temperature, a more exact definition of this might be arrived

at.

387.

How much reason we have in considering these circles to insist on the

_image_ and its effects, is apparent in the phenomenon of the so-called

double suns. Similar double images always occur in certain points

of halos and circles, and only present in a circumscribed form what

takes place in a more general way in the whole circle. All this will

be more conveniently treated in connexion with the appearance of the

rainbow.--Note Q.

388.

In conclusion it is only necessary to point out the affinity between

the catoptrical and paroptical colours.

We call those paroptical colours which appear when the light passes

by the edge of an opaque colourless body. How nearly these are allied

to the dioptrical colours of the second class will be easily seen by

those who are convinced with us that the colours of refraction [Pg 163]

take place only at the edges of objects. The affinity again between the

catoptrical and paroptical colours will be evident in the following

chapter.

XXXII.

PAROPTICAL COLOURS.

389.

The paroptical colours have been hitherto called peri-optical, because

a peculiar effect of light was supposed to take place as it were round

the object, and was ascribed to a certain flexibility of the light to

and from the object.

390.

These colours again may be divided into subjective and objective,

because they appear partly without us, as it were, painted on surfaces,

and partly within us, immediately on the retina. In this chapter we

shall find it more to our purpose to take the objective cases first,

since the subjective are so closely connected with other appearances

already known to us, that it is hardly possible to separate them.

391.

The paroptical colours then are so called because the light must pass

by an outline or edge to produce them. They do not, however, always

appear in this case; to produce the effect very particular conditions

are necessary besides.

392.

It is also to be observed that in this instance again light does not

act as an abstract diffusion (361), the sun shines towards an edge.

The volume of light poured from the sun-image passes by the edge of

a substance, and occasions shadows. Within these shadows we shall

presently find colours appear.

393.

But, above all, we should make the experiments and observations that

bear upon our present inquiry in the fullest light. We, therefore,

place the observer in the open air before we conduct him to the limits

of a dark room.

394.

A person walking in sun-shine in a garden, or on any level path, may

observe that his shadow only appears sharply defined next the foot on

which he rests; farther from this point, especially round the head, it

melts away into the bright ground. For as the sun-light proceeds not

only from the middle of the sun, but also acts cross-wise from the two

extremes of every diameter, an objective parallax takes place which

produces a half-shadow on both sides of the object.

395.

If the person walking raises and spreads his hand, he distinctly sees

in the shadow of each finger the diverging separation of the two

half-shadows outwards, and the diminution of the principal shadow

inwards, both being effects of the cross action of the light.

396.

This experiment may be repeated and varied before a smooth wall,

with rods of different thicknesses, and again with balls; we shall

always find that the farther the object is removed from the surface of

the wall, the more the weak double shadow spreads, and the more the

forcible main shadow diminishes, till at last the main shadow appears

quite effaced, and even the double shadows become so faint, that they

almost disappear; at a still greater distance they are, in fact,

imperceptible.

397.

That this is caused by the cross-action of the light we may easily

convince ourselves; for the shadow of a pointed object plainly exhibits

two points. We must thus never lose sight of the fact that in this

case the whole sun-image acts, produces shadows, changes them to double

shadows, and finally obliterates them.

398.

Instead of solid bodies let us now take openings cut of various given

sizes next each other, and let the sun shine through them on a plane

surface at some little distance; we shall find that the bright image

produced by the sun on the surface, is larger than the opening; this

is because one edge of the sun shines towards the opposite edge of the

opening, while the other edge of the disk is excluded on that side.

Hence the bright image is more weakly lighted towards the edges.

399.

If we take square openings of any size we please, we shall find that

the bright image on a surface nine feet from the opening, is on every

side about an inch larger than the opening; thus nearly corresponding

with the angle of the apparent diameter of the sun.

400.

That the brightness should gradually diminish towards the edges of the

image is quite natural, for at last only a minimum of the light can

act cross-wise from the sun's circumference through the edge of the

aperture.

401.

Thus we here again see how much reason we have in actual observation to

guard against the assumption of parallel rays, bundles and fasces of

rays, and the like hypothetical notions.

402.

We might rather consider the splendour of the sun, or of any light,

as an infinite specular multiplication of the circumscribed luminous

image, whence it may be explained that all square openings through

which the sun shines, at certain distances, according as the apertures

are greater or smaller, must give a round image of light.

403.

The above experiments may be repeated through openings of various

shapes and sizes, and the same effect will always take place at

proportionate distances. In all these cases, however, we may still

observe that in a full light and while the sun merely shines past an

edge, no colour is apparent.

404.

We therefore proceed to experiments with a subdued light, which is

essential to the appearance of colour. Let a small opening be made in

the window-shutter of a dark room; let the crossing sun-light which

enters, be received on a surface of white paper, and we shall find that

the smaller the opening is, the dimmer the light image will be. This is

quite obvious, because the paper does not receive light from the whole

sun, but partially from single points of its disk.

405.

If we look attentively at this dim image of the sun, we find it still

dimmer towards the outlines where a yellow border is perceptible. The

colour is still more apparent if a vapour or a transparent cloud passes

before the sun, thus subduing and dimming its brightness. The halo on

the wall, the effect of the decreasing brightness of a light placed

near it, is here forced on our recollection. (88.)

406.

If we examine the image more accurately, we perceive that this yellow

border is not the only appearance of colour; we can see, besides, a

bluish circle, if not even a halo-like repetition of the coloured

border. If the room is quite dark, we discern that the sky next the

sun also has its effect: we see the blue sky, nay, even the whole

landscape, on the paper, and are thus again convinced that as far as

regards the sun, we have here only to do with a luminous image.

407.

If we take a somewhat larger square opening, so large that the image

of the sun shining through it does not immediately become round, we

may distinctly observe the half-shadows of every edge or side, the

junction of these in the corners, and their colours; just as in the

above-mentioned appearance with the round opening.

408.

We have now subdued a parallactic light by causing it to shine through

small apertures, but we have not taken from it its parallactic

character; so that it can produce double shadows of bodies, although

with diminished power. These double shadows which we have hitherto

been describing, follow each other in light and dark, coloured and

colourless circles, and produce repeated, nay, almost innumerable

halos. These effects have been often represented in drawings and

engravings. By placing needles, hairs, and other small bodies, in the

subdued light, the numerous halo-like double shadows may be increased;

thus observed, they have been ascribed to an alternating flexile action

of the light, and the same assumption has been employed to explain the

obliteration of the central shadow, and the appearance of a light in

the place of the dark.

409.

For ourselves, we maintain that these again are parallactic double

shadows, which appear edged with coloured borders and halos.

410.

After having seen and investigated the foregoing phenomena, we

can proceed to the experiments with knife-blades,[1] exhibiting

effects which may be referred to the contact and parallactic mutual

intersection of the half-shadows and halos already familiar to us.

411.

Lastly, the observer may follow out the experiments with hairs,

needles, and wires, in the half-light produced as before described by

the sun, as well as in that derived from the blue sky, and indicated on

the white paper. He will thus make himself still better acquainted with

the true nature of this phenomenon.

412.

But since in these experiments everything depends on our being

persuaded of the parallactic action of the light, we can make this more

evident by means of two sources of light, the two shadows from which

intersect each other, and may be altogether separated. By day this may

be contrived with two small openings in a window-shutter; by night,

with two candles. There are even accidental effects in interiors, on

opening and closing shutters, by means of which we can better observe

these appearances than with the most careful apparatus. But still,

all and each of these may be reduced to experiment by preparing a box

which the observer can look into from above, and gradually diminishing

the openings after having caused a double light to shine in. In this

case, as might be expected, the coloured shadow, considered under the

physiological colours, appears very easily.

413.

It is necessary to remember, generally, what has been before stated

with regard to the nature of double shadows, half-lights, and the like.

Experiments also should especially be made with different shades of

grey placed next each other, where every stripe will appear light by a

darker, and dark by a lighter stripe next it. If at night, with three

or more lights, we produce shadows which cross each other successively,

we can observe this phenomenon very distinctly, and we shall be

convinced that the physiological case before more fully treated, here

comes into the account (38).

414.

To what extent the appearances that accompany the paroptical colours,

may be derived from the doctrine of subdued lights, from half-shadows,

and from the physiological disposition of the retina, or whether we

shall be forced to take refuge in certain intrinsic qualities of light,

as has hitherto been done, time may teach. Suffice it here to have

pointed out the conditions under which the paroptical colours appear,

and we may hope that our allusion to their connexion with the facts

before adduced by us will not remain unnoticed by the observers of

nature.

415.

The affinity of the paroptical colours with the dioptrical of the

second class will also be readily seen and followed up by every

reflecting investigator. Here, as in those instances, we have to do

with edges or boundaries; here, as in those instances, with a light,

which appears at the outline. How natural, therefore, it is to conclude

that the paroptical effects may be heightened, strengthened, and

enriched by the dioptrical. Since, however, the luminous image actually

shines through the medium, we can here only have to do with objective

cases of refraction: it is these which are strictly allied to the

paroptical cases. The subjective cases of refraction, where we see

objects through the medium, are quite distinct from the paroptical.

We have already recommended them on account of their clearness and

simplicity.

416.

The connexion between the paroptical colours and the catoptrical may

be already inferred from what has been said: for as the catoptrical

colours only appear on scratches, points, steel-wire, and delicate

threads, so it is nearly the same case as if the light shone past an

edge. The light must always be reflected from an edge in order to

produce colour. Here again, as before pointed out, the partial action

of the luminous image and the subduing of the light are both to be

taken into the account.

417.

We add but few observations on the subjective paroptical colours,

because these may be classed partly with the physiological colours,

partly with the dioptrical of the second order. The greater part hardly

seem to belong here, but, when attentively considered, they still

diffuse a satisfactory light over the whole doctrine, and establish its

connexion.

418.

If we hold a ruler before the eyes so that the flame of a light just

appears above it, we see the ruler as it were indented and notched

at the place where the light appears. This seems deducible from the

expansive power of light acting on the retina (18).

419.

The same phenomenon on a large scale is exhibited at sun-rise; for when

the orb appears distinctly, but not too powerfully, so that we can

still look at it, it always makes a sharp indentation in the horizon.

420.

If, when the sky is grey, we approach a window, so that the dark cross

of the window-bars be relieved on the sky; if after fixing the eyes on

the horizontal bar we bend the head a little forward; on half closing

the eyes as we look up, we shall presently perceive a bright yellow-red

border under the bar, and a bright light-blue one above it. The duller

and more monotonous the grey of the sky, the more dusky the room, and,

consequently, the more previously unexcited the eye, the livelier the

appearance will be; but it may be seen by an attentive observer even in

bright daylight.

421.

If we move the head backwards while half closing the eyes, so that the

horizontal bar be seen below, the phenomenon will appear reversed. The

upper edge will appear yellow, the under edge blue.

422.

Such observations are best made in a dark room. If white paper is

spread before the opening where the solar microscope is commonly

fastened, the lower edge of the circle will appear blue, the upper

yellow, even while the eyes are quite open, or only by half-closing

them so far that a halo no longer appears round the white. If the head

is moved backwards the colours are reversed.

423.

These phenomena seem to prove that the humours of the eye are in fact

only really achromatic in the centre where vision takes place, but that

towards the circumference, and in unusual motions of the eyes, as in

looking horizontally when the head is bent backwards or forwards, a

chromatic tendency remains, especially when distinctly relieved objects

are thus looked at. Hence such phenomena may be considered as allied to

the dioptrical colours of the second class.

424.

Similar colours appear if we look on black and white objects, through a

pin-hole in a card. Instead of a white object we may take the minute

light aperture in the tin plate of a camera obscura, as prepared for

paroptical experiments.

425.

If we look through a tube, the farther end of which is contracted or

variously indented, the same colours appear.

426.

The following phenomena appear to me to be more nearly allied to the

paroptical appearances. If we hold up a needle near the eye, the point

appears double. A particularly remarkable effect again is produced if

we look towards a grey sky through the blades of knives prepared for

paroptical experiments. We seem to look through a gauze; a multitude of

threads appear to the eye; these are in fact only the reiterated images

of the sharp edges, each of which is successively modified by the next,

or perhaps modified in a parallactic sense by the oppositely acting

one, the whole mass being thus changed to a thread-like appearance.

427.

Lastly, it is to be remarked that if we look through the blades towards

a minute light in the window-shutter, coloured stripes and halos

appear on the retina as on the paper.

428.

The present chapter may be here terminated, the less reluctantly,

as a friend has undertaken to investigate this subject by further

experiments. In our recapitulation, in the description of the

plates and apparatus, we hope hereafter to give an account of his

observations.[2]

[1] See Newton's Optics, book iii.

[2] The observations here alluded to never appeared.

XXXIII.

EPOPTICAL COLOURS.

429.

We have hitherto had to do with colours which appear with vivacity, but

which immediately vanish again when certain conditions cease. We have

now to become acquainted with others, which it is true are still to be

considered as transient, but which, under certain circumstances, become

so fixed that, even after the conditions which first occasioned their

appearance cease, they still remain, and thus constitute the link

between the physical and the chemical colours.

430.

They appear from various causes on the surface of a colourless body,

originally, without communication, die or immersion (βαφή); and we now

proceed to trace them, from their faintest indication to their most

permanent state, through the different conditions of their appearance,

which for easier survey we here at once summarily state.

431.

First condition.--The contact of two smooth surfaces of hard

transparent bodies.

First case: if masses or plates of glass, or if lenses are pressed

against each other.

Second case: if a crack takes place in a solid mass of glass, chrystal,

or ice.

Third case: if lamellæ of transparent stones become separated.

Second condition.--If a surface of glass or a polished stone is

breathed upon.

Third condition.--The combination of the two last; first, breathing on

the glass, then placing another plate of glass upon it, thus exciting

the colours by pressure; then removing the upper glass, upon which the

colours begin to fade and vanish with the breath.

Fourth condition.--Bubbles of various liquids, soap, chocolate, beer,

wine, fine glass bubbles.

Fifth condition.--Very fine pellicles and lamellæ, produced by the

decomposition of minerals and metals. The pellicles of lime, the

surface of stagnant water, especially if impregnated with iron, and

again pellicles of oil on water, especially of varnish on aqua fortis.

Sixth condition.--If metals are heated; the operation of imparting

tints to steel and other metals.

Seventh condition.--If the surface of glass is beginning to decompose.

432.

First condition, first case. If two convex glasses, or a convex and

plane glass, or, best of all, a convex and concave glass come in

contact, concentric coloured circles appear. The phenomenon exhibits

itself immediately on the slightest pressure, and may then be gradually

carried through various successive states. We will describe the

complete appearance at once, as we shall then be better enabled to

follow the different states through which it passes.

433.

The centre is colourless; where the glasses are, so to speak, united

in one by the strongest pressure, a dark grey point appears with a

silver white space round it: then follow, in decreasing distances,

various insulated rings, all consisting of three colours, which are

in immediate contact with each other. Each of these rings, of which

perhaps three or four might be counted, is yellow on the inner side,

blue on the outer, and red in the centre. Between two rings there

appears a silver white interval. The rings which are farthest from the

centre are always nearer together: they are composed of red and green

without a perceptible white space between them.

434.

We will now observe the appearances in their gradual formation,

beginning from the slightest pressure.

435.

On the slightest pressure the centre itself appears of a green colour.

Then follow as far as the concentric circles extend, red and green

rings. They are wide, accordingly, and no trace of a silver white

space is to be seen between them. The green is produced by the blue of

an imperfectly developed circle, mixing with the yellow of the first

circle. All the remaining circles are, in this slight contact, broad;

their yellow and blue edges mix together, thus producing a beautiful

green. The red, however, of each circle, remains pure and untouched;

hence the whole series is composed of these two colours.

436.

A somewhat stronger pressure separates the first circle by a slight

interval from the imperfectly developed one: it is thus detached, and

may be said to appear in a complete state. The centre is now a blue

point; for the yellow of the first circle is now separated from this

central point by a silver white space. From the centre of the blue a

red appears, which is thus, in all cases, bounded on the outside by

its blue edge. The second and third rings from the centre are quite

detached. Where deviations from this order present themselves, the

observer will be enabled to account for them, from what has been or

remains to be stated.

437.

On a stronger pressure the centre becomes yellow; this yellow is

surrounded by a red and blue edge: at last, the yellow also retires

from the centre; the innermost circle is formed and is bounded with

yellow. The whole centre itself now appears silver white, till at last,

on the strongest pressure, the dark point appears, and the phenomenon,

as described at first, is complete.

438.

The relative size of the concentric circles and their intervals depends

on the form of the glasses which are pressed together.

439.

We remarked above, that the coloured centre is, in fact, an undeveloped

circle. It is, however, often found, on the slightest pressure, that

several undeveloped circles exist there, as it were, in the germ; these

can be successively developed before the eye of the observer.

440.

The regularity of these rings is owing to the form of the convex

glasses, and the diameter of the coloured appearance depends on the

greater or lesser section of a circle on which a lens is polished. We

easily conclude from this, that by pressing plane glasses together,

irregular appearances only will be produced; the colours, in fact,

undulate like watered silks, and spread from the point of pressure in

all directions. Yet, the phenomenon as thus exhibited is much more

splendid than in the former instance, and cannot fail to strike every

spectator. If we make the experiment in this mode, we shall distinctly

see, as in the other case, that, on a slight pressure, the green and

red waves appear; on a stronger, stripes of blue, red, and yellow,

become detached. At first, the outer sides of these stripes touch; on

increased pressure they are separated by a silver white space.

441.

Before we proceed to a further description of this phenomenon, we may

point out the most convenient mode of exhibiting it. Place a large

convex glass on a table near the window; upon this glass lay a plate

of well-polished mirror-glass, about the size of a playing-card, and

the mere weight of the plate will press sufficiently to produce one

or other of the phenomena above described. So, also, by the different

weight of plates of glass, by other accidental circumstances, for

instance, by slipping the plate on the side of the convex glass where

the pressure cannot be so strong as in the centre, all the gradations

above described can be produced in succession.

442.

In order to observe the phenomenon it is necessary to look obliquely

on the surface where it appears. But, above all, it is to be remarked

that by stooping still more, and looking at the appearance under a more

acute angle, the circles not only grow larger but other circles are

developed from the centre, of which no trace is to be discovered when

we look perpendicularly, even through the strongest magnifiers.

443.

In order to exhibit the phenomenon in its greatest beauty, the utmost

attention should be paid to the cleanness of the glasses. If the

experiment is made with plate-glass adapted for mirrors, the glass

should be handled with gloves. The inner surfaces, which must come in

contact with the utmost nicety, may be most conveniently cleaned before

the experiment, and the outer surfaces should be kept clean while the

pressure is increased.

444.

From what has been said it will be seen that an exact contact of two

smooth surfaces is necessary. Polished glasses are best adapted for the

purpose. Plates of glass exhibit the most brilliant colours when they

fit closely together, and for this reason the phenomenon will increase

in beauty if exhibited under an air-pump, by exhausting the air.

445.

The appearance of the coloured rings may be produced in the greatest

perfection by placing a convex and concave glass together which have

been ground on similar segments of circles. I have never seen the

effect more brilliant than with the object-glass of an achromatic

telescope, in which the crown-glass and flint-glass were necessarily

in the closest contact.

446.

A remarkable appearance takes place when dissimilar surfaces are

pressed together; for example, a polished crystal and a plate of

glass. The appearance does not at all exhibit itself in large flowing

waves, as in the combination of glass with glass, but it is small and

angular, and, as it were, disjointed: thus it appears that the surface

of the polished crystal, which consists of infinitely small sections of

lamellæ, does not come so uninterruptedly in contact with the glass as

another glass-plate would.

447.

The appearance of colour vanishes on the strongest pressure, which so

intimately unites the two surfaces that they appear to make but one

substance. It is this which occasions the dark centre, because the

pressed lens no longer reflects any light from this point, for the

very same point, when seen against the light, is perfectly clear and

transparent. On relaxing the pressure, the colours, in like manner,

gradually diminish, and disappear entirely when the surfaces are

separated.

448.

These same appearances occur in two similar cases. If entirely

transparent masses become partially separated, the surfaces of their

parts being still sufficiently in contact, we see the same circles and

waves more or less. They may be produced in great beauty by plunging a

hot mass of glass in water; the different fissures and cracks enabling

us to observe the colours in various forms. Nature often exhibits the

same phenomena in split rock crystals.

449.

This appearance, again, frequently displays itself in the mineral world

in those kinds of stone which by nature have a tendency to exfoliate.

These original lamellæ are, it is true, so intimately united, that

stones of this kind appear altogether transparent and colourless, yet,

the internal layers become separated, from various accidental causes,

without altogether destroying the contact: thus the appearance, which

is now familiar to us by the foregoing description, often occurs in

nature, particularly in calcareous spars; the specularis, adularia, and

other minerals of similar structure. Hence it shows an ignorance of the

proximate causes of an appearance so often accidentally produced, to

consider it so important in mineralogy, and to attach especial value to

the specimens exhibiting it.

450.

We have yet to speak of the very remarkable inversion of this

appearance, as related by men of science. If, namely, instead of

looking at the colours by a reflected light, we examine them by a

transmitted light, the opposite colours are said to appear, and in

a mode corresponding with that which we have before described as

physiological; the colours evoking each other. Instead of blue, we

should thus see red-yellow; instead of red, green, &c., and _vice

versâ_. We reserve experiments in detail, the rather as we have

ourselves still some doubts on this point.

451.

If we were now called upon to give some general explanation of these

epoptical colours, as they appear under the first condition, and to

show their connexion with the previously detailed physical phenomena,

we might proceed to do so as follows:--

452.

The glasses employed for the experiments are to be regarded as the

utmost possible practical approach to transparence. By the intimate

contact, however, occasioned by the pressure applied to them, their

surfaces, we are persuaded, immediately become in a very slight

degree dimmed. Within this semi-transparence the colours immediately

appear, and every circle comprehends the whole scale; for when the two

opposites, yellow and blue, are united by their red extremities, pure

red appears: the green, on the other hand, as in prismatic experiments,

when yellow and blue touch.

453.

We have already repeatedly found that where colour exists at all, the

whole scale is soon called into existence; a similar principle may be

said to lurk in the nature of every physical phenomenon; it already

follows, from the idea of polar opposition, from which an elementary

unity or completeness results.

454.

The fact that a colour exhibited by transmitted light is different

from that displayed by reflected light, reminds us of those dioptrical

colours of the first class which we found were produced precisely in

the same way through semi-opacity. That here, too, a diminution of

transparency exists there can scarcely be a doubt; for the adhesion

of the perfectly smooth plates of glass (an adhesion so strong that

they remain hanging to each other) produces a degree of union which

deprives each of the two surfaces, in some degree, of its smoothness

and transparence. The fullest proof may, however, be found in the

fact that in the centre, where the lens is most strongly pressed on

the other glass, and where a perfect union is accomplished, a complete

transparence takes place, in which we no longer perceive any colour.

All this may be hereafter confirmed in a recapitulation of the whole.

455.

Second condition.--If after breathing on a plate of glass, the breath

is merely wiped away with the finger, and if we then again immediately

breathe on the glass, we see very vivid colours gliding through each

other; these, as the moisture evaporates, change their place, and at

last vanish altogether. If this operation is repeated, the colours are

more vivid and beautiful, and remain longer than they did the first

time.

456.

Quickly as this appearance passes, and confused as it appears to be, I

have yet remarked the following effects:--At first all the principal

colours appear with their combinations; on breathing more strongly, the

appearance may be perceived in some order. In this succession it may be

remarked, that when the breath in evaporating becomes contracted from

all sides towards the centre, the blue colour vanishes last.

457.

The phenomenon appears most readily between the minute lines, which the

action of passing the fingers leaves on the clear surface; a somewhat

rough state of the surface of the glass is otherwise requisite. On

some glass the appearance may be produced by merely breathing; in

other cases the wiping with the fingers is necessary: I have even met

with polished mirror-glasses, one side of which immediately showed the

colours vividly; the other not. To judge from some remaining pieces,

the former was originally the front of the glass, the latter the side

which was covered with quicksilver.

458.

These experiments may be best made in cold weather, because the glass

may be more quickly and distinctly breathed upon, and the breath

evaporates more suddenly. In severe frost the phenomenon may be

observed on a large scale while travelling in a carriage; the glasses

being well cleaned, and all closed. The breath of the persons within is

very gently diffused over the glass, and immediately produces the most

vivid play of colours. How far they may present a regular succession I

have not been able to remark; but they appear particularly vivid when

they have a dark object as a background. This alternation of colours

does not, however, last long; for as soon as the breath gathers in

drops, or freezes to points of ice, the appearance is at once at an end.

459.

Third condition.--The two foregoing experiments of the pressure and

breathing may be united; namely, by breathing on a plate of glass, and

immediately after pressing the other upon it. The colours then appear

as in the case of two glasses unbreathed upon, with this difference,

that the moisture occasions here and there an interruption of the

undulations. On pushing one glass away from the other the moisture

appears iridescent as it evaporates.

460.

It might, however, be asserted that this combined experiment exhibits

no more than each single experiment; for it appears the colours excited

by pressure disappear in proportion as the glasses are less in contact,

and the moisture then evaporates with its own colours.

461.

Fourth condition.--Iridescent appearances are observable in almost all

bubbles; soap-bubbles are the most commonly known, and the effect in

question is thus exhibited in the easiest mode; but it may be observed

in wine, beer, in pure spirit, and again, especially, in the froth of

chocolate.

462.

As in the above cases we required an infinitely narrow space between

two surfaces which are in contact, so we can consider the pellicle

of the soap-bubble as an infinitely thin lamina between two elastic

bodies; for the appearance in fact takes place between the air within,

which distends the bubble, and the atmospheric air.

463.

The bubble when first produced is colourless; then coloured stripes,

like those in marble paper, begin to appear: these at length spread

over the whole surface, or rather are driven round it as it is

distended.

464.

In a single bubble, suffered to hang from the straw or tube, the

appearance of colour is difficult to observe, for the quick rotation

prevents any accurate observation, and all the colours seem to mix

together; yet we can perceive that the colours begin at the orifice of

the tube. The solution itself may, however, be blown into carefully,

so that only one bubble shall appear. This remains white (colourless)

if not much agitated; but if the solution is not too watery, circles

appear round the perpendicular axis of the bubble; these being near

each other, are commonly composed alternately of green and red. Lastly,

several bubbles may be produced together by the same means; in this

case the colours appear on the sides where two bubbles have pressed

each other flat.

465.

The bubbles of chocolate-froth may perhaps be even more conveniently

observed than those of soap; though smaller, they remain longer. In

these, owing to the heat, an impulse, a movement, is produced and

sustained, which appears necessary to the development and succession of

the appearances.

466.

If the bubble is small, or shut in between others, coloured lines

chase each other over the surface, resembling marbled paper; all the

colours of the scale are seen to pass through each other; the pure, the

augmented, the combined, all distinctly clear and beautiful. In small

bubbles the appearance lasts for a considerable time.

467.

If the bubble is larger, or if it becomes by degrees detached, owing

to the bursting of others near, we perceive that this impulsion and

attraction of the colours has, as it were, an end in view; for on

the highest point of the bubble we see a small circle appear, which

is yellow in the centre; the other remaining coloured lines move

constantly round this with a vermicular action.

468.

In a short time the circle enlarges and sinks downwards on all sides;

in the centre the yellow remains; below and on the outside it becomes

red, and soon blue; below this again appears a new circle of the

same series of colours: if they approximate sufficiently, a green is

produced by the union of the border-colours.

469.

When I could count three such leading circles, the centre was

colourless, and this space became by degrees larger as the circles sank

lower, till at last the bubble burst.

470.

Fifth condition.--Very delicate pellicles may be formed in various

ways: on these films we discover a very lively play of colours, either

in the usual order, or more confusedly passing through each other. The

water in which lime has been slaked soon skims over with a coloured

pellicle: the same happens on the surface of stagnant water, especially

if impregnated with iron. The lamellæ of the fine tartar which adheres

to bottles, especially in red French wine, exhibit the most brilliant

colours, on being exposed to the light, if carefully detached. Drops of

oil on water, brandy, and other fluids, produce also similar circles

and brilliant effects: but the most beautiful experiment that can be

made is the following:--Let aqua fortis, not too strong, be poured into

a flat saucer, and then with a brush drop on it some of the varnish

used by engravers to cover certain portions during the process of

biting their plates. After quick commotion there presently appears a

film which spreads itself out in circles, and immediately produces the

most vivid appearances of colour.

471.

Sixth condition.--When metals are heated, colours rapidly succeeding

each other appear on the surface: these colours can, however, be

arrested at will.

472.

If a piece of polished steel is heated, it will, at a certain degree

of warmth, be overspread with yellow. If taken suddenly away from the

fire, this yellow remains.

473.

As the steel becomes hotter, the yellow appears darker, intenser, and

presently passes into red. This is difficult to arrest, for it hastens

very quickly to bright blue.

474.

This beautiful blue is to be arrested if the steel is suddenly taken

out of the heat and buried in ashes. The blue steel works are produced

in this way. If, again, the steel is held longer over the fire, it soon

becomes a light blue, and so it remains.

475.

These colours pass like a breath over the plate of steel; each seems

to fly before the other, but, in reality, each successive hue is

constantly developed from the preceding one.

476.

If we hold a penknife in the flame of a light, a coloured stripe will

appear across the blade. The portion of the stripe which was nearest to

the flame is light blue; this melts into blue-red; the red is in the

centre; then follow yellow-red and yellow.

477.

This phenomenon is deducible from the preceding ones; for the portion

of the blade next the handle is less heated than the end which is in

the flame, and thus all the colours which in other cases exhibited

themselves in succession, must here appear at once, and may thus be

permanently preserved.

478.

Robert Boyle gives this succession of colours as follows:--"A florido

flavo ad flavum saturum et rubescentem (quem artifices sanguineum

vocant) inde ad languidum, postea ad saturiorem cyaneum." This would be

quite correct if the words "languidus" and "saturior" were to change

places. How far the observation is correct, that the different colours

have a relation to the degree of temper which the metal afterwards

acquires, we leave to others to decide. The colours are here only

indications of the different degrees of heat.--Note R.

479.

When lead is calcined, the surface is first greyish. This greyish

powder, with greater heat, becomes yellow, and then orange. Silver,

too, exhibits colours when heated; the fracture of silver in the

process of refining belongs to the same class of examples. When

metallic glasses melt, colours in like manner appear on the surface.

480.

Seventh condition.--When the surface of glass becomes decomposed. The

accidental opacity (blindwerden) of glass has been already noticed: the

term (blindwerden) is employed to denote that the surface of the glass

is so affected as to appear dim to us.

481.

White glass becomes "blind" soonest; cast, and afterwards polished

glass is also liable to be so affected; the bluish less, the green

least.

482.

Of the two sides of a plate of glass one is called the mirror side;

it is that which in the oven lies uppermost, on which one may observe

roundish elevations: it is smoother than the other, which is undermost

in the oven, and on which scratches may be sometimes observed. On this

account the mirror side is placed facing the interior of rooms, because

it is less affected by the moisture adhering to it from within, than

the other would be, and the glass is thus less liable to become "blind."

483.

This half-opacity or dimness of the glass assumes by degrees an

appearance of colour which may become very vivid, and in which perhaps

a certain succession, or otherwise regular order, might be discovered.

484.

Having thus traced the physical colours from their simplest effects to

the present instances, where these fleeting appearances are found to

be fixed in bodies, we are, in fact, arrived at the point where the

chemical colours begin; nay, we have in some sort already passed those

limits; a circumstance which may excite a favourable prejudice for the

consistency of our statement. By way of conclusion to this part of our

inquiry, we subjoin a general observation, which may not be without its

bearing on the common connecting principle of the phenomena that have

been adduced.

485.

The colouring of steel and the appearances analogous to it, might

perhaps be easily deduced from the doctrine of the semi-opaque mediums.

Polished steel reflects light powerfully: we may consider the colour

produced by the heat as a slight degree of dimness: hence a bright

yellow must immediately appear; this, as the dimness increases, must

still appear deeper, more condensed, and redder, and at last pure and

ruby-red. The colour has now reached the extreme point of depth, and

if we suppose the same degree of semi-opacity still to continue, the

dimness would now spread itself over a dark ground, first producing a

violet, then a dark-blue, and at last a light-blue, and thus complete

the series of the appearances.

We will not assert that this mode of explanation will suffice in

all cases; our object is rather to point out the road by which the

all-comprehensive formula, the very key of the enigma, may be at last

discovered.--Note S.