THE double function of the ocular muscles, as described in my letter in NATURE of April 10, p. 422, explains many illusions, one of which has been freely discussed without any satisfactory explanation. This refers to the apparent appearance of a moving motorcar or carriage, as if it were standing still. The eye on moving takes a fresh photograph, which is seen for an appreciable time. The decomposed visual purple is swept out of the way by the back pressure. Note the difference in appearance of a tennis or cricket ball when the eyes are still and when they are moving following the ball. A very curious example may be seen at a cinema. If, when an orator is speaking, the eyes be directed to another part of the picture and then immediately be turned back again to the orator, he will be seen quite still, with his mouth open or closed, while his voice goes on.