The first person who attempted to supply the organ of vision with the luxuries of light and colour, was Father Castel, a learned Jesuit, who had distinguished himself chiefly by his opposition to the splendid optical discoveries of Newton. About the year 1725 or 1726, he published in the _Mercure de Paris_, his first ideas of an organ, or ocular harpsichord. A full account of this curious instrument was afterwards published at Hamburgh, in 1739, by M. Tellemann, a German musician, who had seen one of the harpsichords in the possession of the inventor, when he was on a visit to Paris. This account was afterwards translated into French, and printed at the end of Castel’s _L’Optique des Couleurs_,[18] which appeared at Paris in 1740. The ocular harpsichord is a common harpsichord, fitted up in such a manner, that when a certain sound is produced by striking the keys, a colour related to that sound is at the same instant exhibited to the eye in a box or frame connected with the harpsichord; so that when a piece of music is played for the gratification of the ear, the eye is simultaneously delighted by the display of corresponding colours. [18] This work is entitled, _L’Optique des Couleurs, fondée sur les simples observations et tourné surtout à la pratique de la peinture, de la teinture, et des autres arts coloristes_. Par le R. P. Castel, Jesuite. Paris, 1740. In adjusting the colours and the sounds, Castel lays down the following six propositions:— _1st_,—There is a fundamental and primitive sound in nature, which may be called _ut_, and there is also an original and primitive colour, which is the foundation of all other colours, namely, _blue_. _2d_,—There are three chords, or essential sounds, which depend upon the primitive sound _ut_, and which compose with it a primitive and original accord, and these are _ut_, _mi_, _sol_. There are also three original colours depending on the _blue_, which, while they are not composed of any other colours, produce them all, namely, _blue_, _yellow_, and _red_. The _blue_ is here the note of the tone, the _red_ is the fifth, and the _yellow_ is the third. _3d_,—There are five tonic chords, _ut_, _re_, _mi_, _sol_, _la_, and two semitonic chords, _fa_ and _si_. There are also five tonic colours, to which all the rest are ordinarily related, namely, _blue_, _green_, _yellow_, _red_, and _violet_, and two semitonic or equivocal colours, namely, _aurora_ and _violant_ (related to the _orange_ and _indigo_ of Newton).