From this curious coincidence, it has been proposed to produce a luminous music, consisting of successions or combinations of colours, analogous to a tune in respect to the proportions above mentioned. This might be performed by a strong light, made by means of Mr. Argand's lamps, passing through coloured glasses, and falling on a defined part of the wall, with moveable blinds before them, which might communicate with the keys of a harpsichord, and thus produce at the same time visible and audible music in unison with each other. Now as the pleasure we receive from the sensation of melodious notes, independent of musical time, and of the previous associations of agreeable ideas with them, must arise from our hearing some proportions of sounds after others more easily, distinctly, or agreeably; and as there is a coincidence between the proportions of the primary colours, and the primary sounds, if they may be so called; the same laws must probably govern the sensations of both. In this circumstance therefore consists the sisterhood of Music and Painting; and hence they claim a right to borrow metaphors from each other: musicians to speak of the brilliancy of sounds, and the light and shade of a concerto; and painters of the harmony of colours, and the tone of a picture. This source of pleasure received from the melodious succession of colours or of sounds must not be confounded with the pleasure received from the repetition of them explained above, though the repetition, or division of musical notes into bars, so as to produce common or triple time, contributes much to the pleasure of music; but in viewing a fixed landscape nothing like musical time exists; and the pleasure received therefore from certain successions of colours must depend only on the more easy or distinct action of the retina in perceiving some colours after others, or in their vicinity, like the facility or even pleasure with which we act with contrary muscles in yawning or stretching after having been fatigued with a long previous exertion in the contrary direction. Hence where colours are required to be distinct, those which are opposite to each other, should be brought into succession or vicinity; as red and green, orange and blue, yellow and violet; but where colours are required to intermix imperceptibly, or slide into each other, these should not be chosen; as they might by contrast appear too glaring or tawdry. These gradations and contrasts of colours have been practically employed both by the painters of landscape, and by the planters of ornamental gardens; though the theory of this part of the pleasure derived from visible objects was not explained before the publication of the paper on ocular spectra above mentioned; which is reprinted at the end of the first part of Zoonomia, and has thrown great light on the actions of the nerves of sense in consequence of the stimulus of external bodies. IV. _Association of agreeable sentiments with visible objects._