VIII.

SUBJECTIVE HALOS.

89.

Halos may be divided into subjective and objective. The latter will be considered under the physical colours; the first only belong here. These are distinguished from the objective halos by the circumstance of their vanishing when the point of light which produces them on the retina is covered.

90.

We have before noticed the impression of a luminous object on the retina, and seen that it appears larger: but the effect is not at an end here, it is not confined to the impression of the image; an expansive action also takes place, spreading from the centre.

91.

That a nimbus of this kind is produced round the luminous image in the eye may be best seen in a dark room, if we look towards a moderately large opening in the window-shutter. In this case the bright image is surrounded by a circular misty light. I saw such a halo bounded by a yellow and yellow-red circle on opening my eyes at dawn, on an occasion when I passed several nights in a bed-carriage.

92.

Halos appear most vivid when the eye is susceptible from having been in a state of repose. A dark background also heightens their appearance. Both causes account for our seeing them so strong if a light is presented to the eyes on waking at night. These conditions were combined when Descartes after sleeping, as he sat in a ship, remarked such a vividly-coloured halo round the light.

93.

A light must shine moderately, not dazzle, in order to produce the impression of a halo in the eye; at all events the halos of dazzling lights cannot be observed. We see a splendour of this kind round the image of the sun reflected from the surface of water.

94.

A halo of this description, attentively observed, is found to be encircled towards its edge with a yellow border: but even here the expansive action, before alluded to, is not at an end, but appears still to extend in varied circles.

95.

Several cases seem to indicate a circular action of the retina, whether owing to the round form of the eye itself and its different parts, or to some other cause.

96.

If the eye is pressed only in a slight degree from the inner corner, darker or lighter circles appear. At night, even without pressure, we can sometimes perceive a succession of such circles emerging from, or spreading over, each other.

97.

We have already seen that a yellow border is apparent round the white space illumined by a light placed near it. This may be a kind of objective halo. (88.)

98.

Subjective halos may be considered as the result of a conflict between the light and a living surface. From the conflict between the exciting principle and the excited, an undulating motion arises, which may be illustrated by a comparison with the circles on water. The stone thrown in drives the water in all directions; the effect attains a maximum, it reacts, and being opposed, continues under the surface. The effect goes on, culminates again, and thus the circles are repeated. If we have ever remarked the concentric rings which appear in a glass of water on trying to produce a tone by rubbing the edge; if we call to mind the intermitting pulsations in the reverberations of bells, we shall approach a conception of what may take place on the retina when the image of a luminous object impinges on it, not to mention that as a living and elastic structure, it has already a circular principle in its organisation.--Note G.

99.

The bright circular space which appears round the shining object is yellow, ending in red: then follows a greenish circle, which is terminated by a red border. This appears to be the usual phenomenon where the luminous body is somewhat considerable in size. These halos become greater the more distant we are from the luminous object.

100.

Halos may, however, appear extremely small and numerous when the impinging image is minute, yet powerful, in its effect. The experiment is best made with a piece of gold-leaf placed on the ground and illumined by the sun. In these cases the halos appear in variegated rays. The iridescent appearance produced in the eye when the sun pierces through the leaves of trees seems also to belong to the same class of phenomena.

PATHOLOGICAL COLOURS.

APPENDIX.

101.

We are now sufficiently acquainted with the physiological colours to distinguish them from the pathological. We know what appearances belong to the eye in a healthy state, and are necessary to enable the organ to exert its complete vitality and activity.

102.

Morbid phenomena indicate in like manner the existence of organic and physical laws: for if a living being deviates from those rules with reference to which it is constructed, it still seeks to agree with the general vitality of nature in conformity with general laws, and throughout its whole course still proves the constancy of those principles on which the universe has existed, and by which it is held together.

103.

We will here first advert to a very remarkable state in which the vision of many persons is found to be. As it presents a deviation from the ordinary mode of seeing colours, it might be fairly classed under morbid impressions; but as it is consistent in itself, as it often occurs, may extend to several members of a family, and probably does not admit of cure, we may consider it as bordering only on the nosological cases, and therefore place it first.

104.

I was acquainted with two individuals not more than twenty years of age, who were thus affected: both had bluish-grey eyes, an acute sight for near and distant objects, by day-light and candle-light, and their mode of seeing colours was in the main quite similar.

105.

They agreed with the rest of the world in denominating white, black, and grey in the usual manner. Both saw white untinged with any hue. One saw a somewhat brownish appearance in black, and in grey a somewhat reddish tinge. In general they appeared to have a very delicate perception of the gradations of light and dark.

106.

They appeared to see yellow, red-yellow, and yellow-red,[1] like others: in the last case they said they saw the yellow passing as it were over the red as if glazed: some thickly-ground carmine, which had dried in a saucer, they called red.

107.

But now a striking difference presented itself. If the carmine was passed thinly over the white saucer, they would compare the light colour thus produced to the colour of the sky, and call it blue. If a rose was shown them beside it, they would, in like manner, call it blue; and in all the trials which were made, it appeared that they could not distinguish light blue from rose-colour. They confounded rose-colour, blue, and violet on all occasions: these colours only appeared to them to be distinguished from each other by delicate shades of lighter, darker, intenser, or fainter appearance.

108.

Again they could not distinguish green from dark orange, nor, more especially, from a red brown.

109.

If any one, accidentally conversing with these individuals, happened to question them about surrounding objects, their answers occasioned the greatest perplexity, and the interrogator began to fancy his own wits were out of order. With some method we may, however, approach to a nearer knowledge of the law of this deviation from the general law.

110.

These persons, as may be gathered from what has been stated, saw fewer colours than other people: hence arose the confusion of different colours. They called the sky rose-colour, and the rose blue, or _vice versâ_. The question now is: did they see both blue or both rose-colour? did they see green orange, or orange green?

111.

This singular enigma appears to solve itself, if we assume that they saw no blue, but, instead of it, a light pure red, a rose-colour. We can comprehend what would be the result of this by means of the chromatic diagram.

112.

If we take away blue from the chromatic circle we shall miss violet and green as well. Pure red occupies the place of blue and violet, and in again mixing with yellow the red produces orange where green should be.

113.

Professing to be satisfied with this mode of explanation, we have named this remarkable deviation from ordinary vision "Acyanoblepsia."[2] We have prepared some coloured figures for its further elucidation, and in explaining these we shall add some further details. Among the examples will be found a landscape, coloured in the mode in which the individuals alluded to appeared to see nature: the sky rose-colour, and all that should be green varying from yellow to brown red, nearly as foliage appears to us in autumn[3].--Note H.

114.

We now proceed to speak of morbid and other extraordinary affections of the retina, by which the eye may be susceptible of an appearance of light without external light, reserving for a future occasion the consideration of galvanic light.

115.

If the eye receives a blow, sparks seem to spread from it. In some states of body, again, when the blood is heated, and the system much excited, if the eye is pressed first gently, and then more and more strongly, a dazzling and intolerable light may be excited.

116.

If those who have been recently couched experience pain and heat in the eye, they frequently see fiery flashes and sparks: these symptoms last sometimes for a week or fortnight, or till the pain and heat diminish.

117.

A person suffering from ear-ache saw sparks and balls of light in the eye during each attack, as long as the pain lasted.

118.

Persons suffering from worms often experience extraordinary appearances in the eye, sometimes sparks of fire, sometimes spectres of light, sometimes frightful figures, which they cannot by an effort of the will cease to see: sometimes these appearances are double.

119.

Hypochondriacs frequently see dark objects, such as threads, hairs, spiders, flies, wasps. These appearances also exhibit themselves in the incipient hard cataract. Many see semi-transparent small tubes, forms like wings of insects, bubbles of water of various sizes, which fall slowly down, if the eye is raised: sometimes these congregate together so as to resemble the spawn of frogs; sometimes they appear as complete spheres, sometimes in the form of lenses.

120.

As light appeared, in the former instances, without external light, so also these images appear without corresponding external objects. The images are sometimes transient, sometimes they last during the patient's life. Colour, again, frequently accompanies these impressions: for hypochondriacs often see yellow-red stripes in the eye: these are generally more vivid and numerous in the morning, or when lasting.

121.

We have before seen that the impression of any object may remain for a time in the eye: this we have found to be a physiological phenomenon (23): the excessive duration of such an impression, on the other band, may be considered as morbid.

122.

The weaker the organ the longer the impression of the image lasts. The retina does not so soon recover itself; and the effect may be considered as a kind of paralysis (28).

123.

This is not to be wondered at in the case of dazzling lights. If any one looks at the sun, he may retain the image in his eyes for several days. Boyle relates an instance of ten years.

124.

The same takes place, in a certain degree, with regard to objects that are not dazzling. Büsch relates of himself that the image of an engraving, complete in all its parts, was impressed on his eye for seventeen minutes.

125.

A person inclined to fulness of blood retained the image of a bright red calico, with white spots, many minutes in the eye, and saw it float before everything like a veil. It only disappeared by rubbing the eye for some time.

126.

Scherfer observes that the red colour, which is the consequence of a powerful impression of light, may last for some hours.

127.

As we can produce an appearance of light on the retina by pressure on the eyeball, so by a gentle pressure a red colour appears, thus corresponding with the after-image of an impression of light.

128.

Many sick persons, on awaking, see everything in the colour of the morning sky, as if through a red veil: so, if in the evening they doze and wake again, the same appearance presents itself. It remains for some minutes, and always disappears if the eye is rubbed a little. Red stars and balls sometimes accompany the impression. This state may last for a considerable time.

129.

The aëronauts, particularly Zambeccari and his companions, relate that they saw the moon blood-red at the highest elevation. As they had ascended above the vapours of the earth, through which we see the moon and sun naturally of such a colour, it may be suspected that this appearance may be classed with the pathological colours. The senses, namely, may be so influenced by an unusual state, that the whole nervous system, and particularly the retina, may sink into a kind of inertness and inexcitability. Hence it is not impossible that the moon might act as a very subdued light, and thus produce the impression of the red colour. The sun even appeared blood-red to the aëronauts of Hamburgh.

If those who are at some elevation in a balloon scarcely hear each other speak, may not this, too, be attributed to the inexcitable state of the nerves as well as to the thinness of the air?

130.

Objects are often seen by sick persons in variegated colours. Boyle relates an instance of a lady, who, after a fall by which an eye was bruised, saw all objects, but especially white objects, glittering in colours, even to an intolerable degree.

131.

Physicians give the name of "Chrupsia" to an affection of the sight, occurring in typhoid maladies. In these cases the patients state that they see the boundaries of objects coloured where light and dark meet. A change probably takes place in the humours of the eye, through which their achromatism is affected.

132.

In cases of milky cataract, a very turbid crystalline lens causes the patient to see a red light. In a case of this kind, which was treated by the application of electricity, the red light changed by degrees to yellow, and at last to white, when the patient again began to distinguish objects. These changes of themselves warranted the conclusion that the turbid state of the lens was gradually approaching the transparent state. We shall be enabled easily to trace this effect to its source as soon as we become better acquainted with the physical colours.

133.

If again it may be assumed that a jaundiced patient sees through an actually yellow-coloured humour, we are at once referred to the department of chemical colours, and it is thus evident that we can only thoroughly investigate the chapter of pathological colours when we have made ourselves acquainted with the whole range of the remaining phenomena. What has been adduced may therefore suffice for the present, till we resume the further consideration of this portion of our subject.

134.

In conclusion we may, however, at once advert to some peculiar states or dispositions of the organ.

There are painters who, instead of rendering the colours of nature, diffuse a general tone, a warm or cold hue, over the picture. In some, again, a predilection for certain colours displays itself; in others a want of feeling for harmony.

135.

Lastly, it is also worthy of remark, that savage nations, uneducated people, and children have a great predilection for vivid colours; that animals are excited to rage by certain colours; that people of refinement avoid vivid colours in their dress and the objects that are about them, and seem inclined to banish them altogether from their presence.--Note I.

[1] It has been found necessary to follow the author's nomenclature throughout--T.

[2] Non-perception of blue.

[3] It has not been thought necessary to copy the plates here referred to.--T.

## PART II.

PHYSICAL COLOURS.

136.

We give this designation to colours which are produced by certain material mediums: these mediums, however, have no colour themselves, and may be either transparent, semi-transparent yet transmitting light, or altogether opaque. The colours in question are thus produced in the eye through such external given causes, or are merely reflected to the eye when by whatever means they are already produced without us. Although we thus ascribe to them a certain objective character, their distinctive quality still consists in their being transient, and not to be arrested.

137.

They are called by former investigators _colores apparentes, fluxi, fugitivi, phantastici, falsi, variantes_. They are also called _speciosi_ and _emphatici_, on account of their striking splendour. They are immediately connected with the physiological colours, and appear to have but little more reality: for, while in the production of the physiological colours the eye itself was chiefly efficient, and we could only perceive the phenomena thus evoked within ourselves, but not without us, we have now to consider the fact that colours are produced in the eye by means of colourless objects; that we thus too have a colourless surface before us which is acted upon as the retina itself is, and that we can perceive the appearance produced upon it without us. In such a process, however, every observation will convince us that we have to do with colours in a progressive and mutable, but not in a final or complete, state.

138.

Hence, in directing our attention to these physical colours, we find it quite possible to place an objective phenomenon beside a subjective one, and often by means of the union of the two successfully to penetrate farther into the nature of the appearance.

139.

Thus, in the observations by which we become acquainted with the physical colours, the eye is not to be considered as acting alone; nor is the light ever to be considered in immediate relation with the eye: but we direct our attention especially to the various effects produced by mediums, those mediums being themselves colourless.

140.

Light under these circumstances may be affected by three conditions. First, when it flashes back from the surface of a medium; in considering which _catoptrical_ experiments invite our attention. Secondly, when it passes by the edge of a medium: the phenomena thus produced were formerly called _perioptical_; we prefer the term _paroptical_. Thirdly, when it passes through either a merely light-transmitting or an actually transparent body; thus constituting a class of appearances on which _dioptrical_ experiments are founded. We have called a fourth class of physical colours _epoptical_, as the phenomena exhibit themselves on the colourless surface of bodies under various conditions, without previous or actual dye (βαφή).--Note K.

141.

In examining these categories with reference to our three leading divisions, according to which we consider the phenomena of colours in a physiological, physical, or chemical view, we find that the catoptrical colours are closely connected with the physiological; the paroptical are already somewhat more distinct and independent; the dioptrical exhibit themselves as entirely and strictly physical, and as having a decidedly objective character; the epoptical, although still only apparent, may be considered as the transition to the chemical colours.

142.

If we were desirous of prosecuting our investigation strictly in the order of nature, we ought to proceed according to the classification which has just been made; but in didactic treatises it is not of so much consequence to connect as to duly distinguish the various divisions of a subject, in order that at last, when every single class and case has been presented to the mind, the whole may be embraced in one comprehensive view. We therefore turn our attention forthwith to the dioptrical class, in order at once to give the reader the full impression of the physical colours, and to exhibit their characteristics the more strikingly.