We may thus easily account for that temporary blindness which follows gazing on the sun or & powerful light, by which the principles of vision become exhausted. Thus also that kind of nycta-lophia which occurs in tropical climates, which comes on regularly at the close of day, and goes off when it advances, appears to arise from defective secretion or excessive exhaustion, being usually attributed either to disorder of the digestive organs, or the power of the sun's light, and is cured or relieved by secluding the eyes from light during the day; - while that kind of nyctalophia called moon-eyed, which is common to the bushmen of Southern Africa, who sleep during the day, and are blind when the sun shines, but who, like feline animals, see well in seeming darkness, may be supposed to arise from redundant secretion or defective excretion of the principles of light and vision.

We may in like manner explain also that morbid sensibility of the retina, in which hot colours and strong contrasts become intolerable to the eye, as owing to unequal secretion or an inflamed state of the organ. We perceive herefrom why also the brilliancy of colours declines upon long viewing them, particularly in a strong light. A knowledge of all these powers and effects of light and colours is hardly less interesting and essential to the medical practitioner than to the artist, if he would form a correct judgment of health and disease by their means. In fine, the physiologist may hence take a hint toward a physical explanation of all sensation in the nervous system, and of the union of all sensible impressions in the sensorium or brain, as a link of identity or connexion between the physical and metaphysical world.

It is worthy of remark here, that the phosphori in general emit light of the same colour as that to which they have been exposed.

Newton remarked, also, that inflammable or hydrogenous substances refract light more powerfully than other substances, and that the diamond does so most of all; whence he framed the admirable conjecture, since proved, that the diamond itself is inflammable. By a like analogy we may infer, that since non-inflammable substances refract light but weakly, and with faint colours, it is probable that the oxygenous principle predominates therein, or that they are oxides, which accords with the discoveries of Davy; and, as various substances have not only various powers by which they refract or bend more or less the course of light, but also various powers by which they disperse or separate the colours of light, we are not aware of any mode of accounting for these effects in so adequate and simple a manner, as by the chemistry of light and colours here advanced. But we are getting beyond the purposes of painting.

Colour, and what in painting is called transparency, belong principally we perceive to shade; and the judgment of great authorities, by which they have been attached to light as its properties merely, has led to error in an art to which colour is pre-eminently appropriate; - hence the painter has considered colour in his practice as belonging to light only, and hence many have employed a uniform shade tint, regarding shadows only as darkness, blackness, or the mere absence of light, when in truth shadows are infinitely varied by colour, and always so by the colours of the lights which produce them. We must, however, avoid conducting to a vicious extreme, while we incline attention toward the relation of colour to shade, both light and shade being in strictness coessentiat to colour; but, as transparent, colour inclines to shade, and as opaque it partakes of light, yet the general tendency of colour is to transparency and shade, all colour being a departure from light. It hence becomes a maxim, which he who aspires to good colouring must never lose sight of, that the colour of shadow is always transparent, and that of extreme light objects only opaque. It follows that white is to be kept as much as possible out of shadow, and black, for the same reason, out of colour, employing opaque tints in each case, instead of black or white, whenever it is necessary to cover, and glazing them with transparent colours. Such practice would also be more favourable to durability of the tones of pictures than the shades and tints produced with black and white. The hues and shadows of Nature are in no ordinary case either black or white, which are always poor, frigid, and fearful in colouring, except as local colours.

For this doctrine we have also the high authority of Rubens, who, in the following extract from his Lessons, says, " Begin by painting in your shadows lightly, taking particular care that no white is suffered to glide into them; it is the poison of a picture, except in the lights: if once your shadows are corrupted by the introduction of this baneful colour, your tones will be no longer warm and transparent, but heavy and leady. It is not the same in the lights; they may be loaded with [opaque] colour as much as you may think proper, provided the tones are kept pure: you are sure to succeed in placing each tint in its place, and afterwards by a light blending with the brush or pencil, melting them into each other without tormenting them, and on this preparation may be given those decided touches which are always the distinguishing marks of the great master;" and, although in modes of practice differing from that of Rubens, the contrary of this precept has been followed, yet, as it applies only to the upper painting, the principle itself is universal.

It is to be noted also, that the colour of shadow is always complementary to that of its light, modified by the local colours upon which it falls; and this accords equally with correct observation and the foregoing principles, although these are at variance with the ordinary practice of artists.

Of the mechanical, dynamical, and optical relations of light and shade, so far as regards painting and colours, we need only briefly remark, that the motion or action of light is either direct, reflected, or inflected; - that the direct lights of the sun and moon are always in straight lines nearly parallel to each other; - that artificial lights diverge from themselves as centres in radii, and all light partakes of the colour of the medium through which it passes; - that of reflected light, the angles of reflection are always equal to the angles of incidence, and partake of the colours of the reflected surfaces; - and, respecting refracted lights, that in passing through transparent media, or by opaque objects, light, whether direct or reflected, is always inflected with a developement also of much or little colour; and that the shadows of light in each case is always the chromatic equivalent of such light. Opticians regard the motion of the sun's light as propagated in parallel rays, and attribute the like parallelism to other lights, abating always the diameter of the agents, and this may be very allowable as a mathematical fiction, but cannot be maintained as a fact; for light is an infinite agent, diffusing itself expansively from every point and particle, till utterly expanded or expended in darkness, the patient of light, according to its various affinities: and to this we owe the penumbrae of shadows, and all the effects of transient colours from prisms, lenses, and transparent solids - and of colours meteorological and spectral.

In passing an opaque object, light is always bent or inflected toward, or into, its shadow, and the shadow bends into the light; consequently, there is a penumbra surrounding every shade, forming a softening medium between it and the light, and aiding reflection in enlightening every shade; every light has hence its shade, and every shadow its light. It is in the management of these properties of light that the skill of the artist is no less requisite and conspicuous than in the management of colours, with which they are intimately connected.

To conclude, - we know not whether the preceding attempt to explain the causes and effects of vision, light, and colours, physically or chemically, may prove satisfactory to other minds; but of this we feel assured, that the first elements of things are powers and not particles; that the modern corpuscular and undulatory doctrines, with all the mathematical and mechanical explanations hitherto employed, are entirely incompetent to the solution of these phenomena; and that all the hypotheses built upon them, like those which they superseded,* must ultimately fail at the foundation, not even answering the inquiry of the poet: -

"Why does one climate and one soil endue The blushing poppy with an orange hue, Yet leave the lily pale, and tinge the violet blue?"

Prior.

* OF the best of these was that often quoted of Empedocles, the Pythagorean, from which school has emanated some of the most refined and important of antient and modern systems; - but with this hypothesis, which accounted for vision by the emission of light from the eye, the powerful mind of Socrates declared itself not. thoroughly satisfied; indeed, it is the extreme opposed to common sense and modem doctrine (as was also the solar system of Pythagoras), and truth lies not in extremes but in a medium which connects them. - See Sydenham's "Plato; Meno," p. 75.