Newton's Optics, Book I. part 2. prop. 3 and 6. Dr. Smith, in his Harmonics, has an explanatory note upon this happy discovery, as he terms it, of Newton. Sect. 4. Art. 7. From this curious coincidence, it has been proposed to produce a luminous music, confiding 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 a 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. The execution of this idea is said by Mr. Guyot to have been attempted by Father Cassel without much success. If this should be again attempted, there is another curious coincidence between sounds and colours, discovered by Dr. Darwin of Shrewsbury, and explained in a paper on what he calls Ocular Spectra, in the Philosophical Transactions, Vol. LXXVI. which might much facilitate the execution of it. In this treatise the Doctor has demonstrated, that we see certain colours, not only with greater ease and distinctness, but with relief and pleasure, after having for some time contemplated other certain colours; as green after red, or red after green; orange after blue, or blue after orange; yellow after violet, or violet after yellow. This he shews arises from the _ocular spectrum_ of the colour last viewed coinciding with the _irritation_ of the colour now under contemplation. Now as the pleasure we receive from the sensation of melodious notes, independent 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; he argues, that the same laws must 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. Thus it was not quite so absurd, as was imagined, when the blind man asked if the colour scarlet was like the sound of a trumpet. As the coincidence or opposition of these _ocular spectra_, (or colours which remain in the eye after having for some time contemplated a luminous object) are more easily and more accurately ascertained, now their laws have been investigated by Dr. Darwin, than the _relicts_ of evanescent sounds upon the ear; it is to be wished that some ingenious musician would further cultivate this curious field of science: for if visible music can be agreeably produced, it would be more easy to add sentiment