72

be in the power of the will to determine the appearance of either of the letters.”

A second experiment, in which different colours are employed, is also brought forward by Professor Wheatstone, to prove that the mind cannot at the same time perceive two dissimilar pictures, such pictures being simultaneously presented to corresponding parts of the two retinæ:

“If a blue disc is presented to the right eye, and a yellow disc to the corresponding part of the left eye, instead of a green disc, which would appear if these two colours had mingled before their arrival at a single eye, the mind will perceive these two colours distinctly, one or the other alternately predominating, either partially or wholly oveer the disc. In the same manner the mind perceives no trace of violet when red is presented to one eye and blue to the other, nor any vestige of orange when red and yellow are separately presented in a similar manner.”