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Physiology of vision,” published in the ‘Philosophical Transactions of the Royal Society of London’ for 1838.

In this memoir Professor Wheatstone adduces two principal experiments - one of them refers to the phenomena which may be observed, when dissimilar objects are simultaneously presented to analogous parts of the two retinæ, and the other to the confluence of two unequal images, the question in both cases being, are the results obtained to be ascribed to the mind, or do they arise from causes purely structural?

If the former, they carry with them very important physiological conclusions; if the latter, they are interesting phenomena.

The experiment to which I shall first allude is brought forward to prove, that if dissimilar objects be simultaneously presented to the two eyes respectively, the mind will not recognise both impressions at the same time; neither will the two dissimilar images coalesce or become permanently superposed; but that now one, then the other, will preponderate, or fragments of both will appear, the two being seen together only at the moment of interchange.

The form of the experiment adopted by Professor Wheatstone in support of his conclusion is as follows:

“If A and B are each presented at the same time to a different eye, the common border will remain constant, while the letter within it will change alternately from that which would be perceived by the right eye alone to that which would be perceived by the left eye alone. At the moment of change the letter which has just been seen breaks into fragments, while fragments of the letter which is about to appear mingle with them, and are immediately after replaced by the entire letter. It does not appear,” adds Professor Wheatstone, “to