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of being such as can be easily repeated by means of the ordinary stereoscope, and by any person sufficiently interested in the subject.

If a number of parallel lines be drawn on two pieces of paper, in size not exceeding one inch square, on the one square the lines are to be drawn at equal distances, and with a steel pen, that they may be fine and of uniform thickness. On the other square the lines are to be drawn differing somewhat in distance, and of unequal thickness. (Fig. 1.)

If these two squares be gummed upon a piece of card, and presented to the corresponding parts of the two retinæ, with the lines in a vertical direction for one eye, and in the horizontal direction for the other eye, the result will be precisely that described by Professor Wheatstone in the experiment A and B, that is, the two figures will appear to break into fragments, and they will alternate.

If, however, the experiment be repeated with slight modifi-