To this successive modification of states of an organ, terminating in a particular result, different from each of the prior states, there are abundant analogies in the history of the mind, and many in the phenomena of sensation itself. One of the most remarkable of these is the production of the sensation of _whiteness_, by the rapid revolution of a cylinder, on which the separate prismatic colours, and the separate colours only, are painted, in certain proportions; _each colour_, in this case, acting on the organ already affected by a former colour, till a sensation altogether different from the result of each of them when separate, is their joint ultimate result, the sensation of _whiteness_, without any external object that is white. In this way only, by a series of progressive organic affections, and not by any single affection, can the vibration of an elastic medium, as different from one simple unrepeated impulse, terminate in the production of sound. It is, in short, a name for this series of changes, and nothing more. If, in a case so very obscure as that of _musical ear_, in which all that is truly evident, is, that in different individuals, there is a diversity of some kind or other--I could permit myself to indulge any conjecture with respect to this diversity,--I might perhaps, be inclined to look to the view now given of the real nature of _vibration_, and its progressive effects on the auditory part of our nervous system, as furnishing some slight ground, not, indeed, for any theory, which is far too presumptuous a word, but for the preference of one mere possibility, to other mere possibilities, which is all that can be hoped in any conjecture, on so very dim and impalpable a subject. We have seen that the series of pulses of the vibrating air,--if vibration, or a series of pulses be necessary to sound,--must produce a series of changes in the sensorial organ, which produce no corresponding affection of the mind, till, at last, a state of the organ is produced, which _is_ attended with sensation. This, and this only, can be meant, when we speak of vibration as the antecedent of sound,--a series of organic changes, and, after this series, an affection of the mind. In such circumstances, it is certainly more probable, that the organ thus affected with a series of progressive changes, does not pass instantly from the greatest change to the state in which it was originally, before the first pulse, but that it retains this state, for a time, however, short, or, at least, passes through some series of states, in its gradual return, so that, if a _new_ vibration be excited by the pulse of any sounding body, before the organ of hearing have returned to its original state, the effect may be supposed to be different from that which it would have been, if the same vibration had been primarily communicated to the organ, in its state of rest, or in that state, which, from our want of a better word, may be termed its state of rest.