"The mind of Leonardo was, however, too active and capacious to be contented solely with the practical part of his art; nor could it submit to receive as principles, conclusions, though confirmed by experience, without first tracing them to their source, and investigating their causes, and the several circumstances on which they depended." - Hawkins's Life of Da Vinci.

In treating of pigments, there is no speculation more natural than the inquiry, What are the physical causes of colour in general? We shall not, therefore, pass it over unnoticed, more especially because the right explanation of many effects and appearances, both of colour and vision, in which the artist is concerned depend upon its solution, though it is a question of undoubted difficulty, which has never been satisfactorily answered; being perhaps too abstract and elementary to admit of sensible demonstration, and therefore incapable of popular solution. Hence the theory of colours has varied with the changes of philosophy, and the fashions of the mind.

According to common apprehension, colours are inherent or substantial qualities of bodies, or material things.* The more speculative have ascribed them to sense or vision only, regarding them as intellectual affections through the organs of sight; while a third class of inquirers has assigned them variously a medial station between vision and coloured objects: viz. in light.

Each of these doctrines has been sanctioned by great authorities, and they have given birth to the various hypotheses upon which the theories of colours have been founded and compounded; but, however various and ingenious these theories may have been, none has hitherto fulfilled the strict requisitions of reason and experience.

It is probable, and perhaps demonstrable, that colours in the abstract belong neither exclusively to the object, the subject, nor the medium, but refer in equal relation to them all; while they are capable of being regarded in either respect; physically regarded, they are material objects, - metaphysically, they are intellectual impressions, - and, in an aethetical sense, they are sensible representations of material objects to the mind.

Our present question is, however, purely physical, or that of the natural philosopher; in which view colours are either inherent, as belonging to their passive objects, as in pigments, etc, or transient, as belonging to their concurrent medium, as in light, ocular spectra, etc. And in both they are here to be regarded as material and of the same nature; but the colours of light being the most pure, simple, and elementary, have afforded the most usual and eligible basis for this inquiry.

* It may perhaps be ultimately found, in this as in other cases, that nature does not play the Cool with our senses, but that the last accomplishments of science coincide with common apprehension.

Yet sensible light is not a simple substance, but an effect of the concurrence of two elementary powers: one of which is the active principle of light; the other, passive or re-active, and to be regarded as the principle of shade, or darkness, - the first, coincident if not identical with the oxygen of the chemist; the other, with hydrogen;* and, however exceptionable this may be to those who have been accustomed to regard darkness as a mere privation of light, yet, as respects the artist, a principle of darkness, blackness, and shade, is as essential as is a principle of light.

Accordingly the sunbeam, as it arrives to us, is a, compound of these elements of light and shade, and it may be analyzed by refraction, and in other ways, into oxidizing or whitening rays, and hydro-genizing or blackening rays; and at the same time into others that are variously compounded of these, and variously coloured.

* We adopt these terms for the two principles of light, not for their fitness, but because they have already been used in a similar elementary sense by the chemists; - otherwise either electrogen and thamogen, or phosphagen and sciogen, were more analogous terms: and it might be well that natural philosophers should agree upon general appellations for the two opposed or concurrent principles of light, - of electricity, - of galvanism, - of magnetism, and of every chemical or physical elementary science, since there can now be little doubt of their original identity; the experimental demonstration of which, and of our physical rationale of colours, is beside our present purpose, and pregnant with materials for volumes.

Light hence appears, as before remarked, to be in the sunbeam the effect of the concurrence or conjugation of two aethereal, electrical, or elementary substances or powers: the one, an agent, of which the sun appears to be the fountain or source; the other, a re-agent, existing in planetary or atmospheric space, analogous to shade; - if so, the sun's light is a species of oxidation or combustion, a sort of flame attended by sensible or latent heat, and all light must be regarded as similarly constituted and produced by the active uniting of an oxygenous or electral principle with a phlogistic or thermal principle.

Light has nevertheless been considered, equally by the common and philosophic observers, as single or simple, and not as containing in itself any antagonist principles, but as having merely intension and remission.

Newton was the first who taught to regard the sunbeam as a compound of rays of various powers and colours, but still he regarded it singly, and its heat as accidental. Subsequent investigation has shewn that his analysis was chemically defective; - he erred also in regarding light as a compound of heterogeneous rays, of colours and powers essentially different; and in other respects, important to art on account of the great authority of his name.*

Scheele and others have demonstrated rays of an invisible kind, accompanying the colours of the sunbeam in the prismatic spectrum, which have been denominated variously deoxidizing, chemical, and phlogistic, or hydrogenizing rays, † and prove the existence of a tenebrous or dark principle, by which light is modified. Herschel has also investigated the calorific rays, or heat of the sunbeam; but these are to be regarded, with Newton, rather as accidental, or an effect of another sense, than as a constituent or principle of light: thus we have real and demonstrable principles in place of hypothesis, upon which to explain the various phenomena of light and colours.