Page:Popular Science Monthly Volume 6.djvu/412

396 the star, so as to excite the nerves of vision, are both very small: it is the coördinating power of the brain that works slowly—and absolute personal equation is largely the measure of the time required for the brain to superpose two different sensations, to coördinate impressions derived from different sets of nerves.

This view M. Wolf combats, and maintains, on the contrary, that the phenomenon in question is purely physiological, and arises from the duration of the luminous impression of the image of the star on the retina. To prove this, he has applied his apparatus to the observing of transits in which the seconds of the clock were not marked by audible beats, but by flashes of light appearing in the field of the telescope.

In this case, and also in the case where the seconds of the clock were not heard, but were marked by light taps on his hand, his equation remained almost constant (see table):

This table seems to bear out M. Wolf’s view; but, in this connection, it will be interesting to refer to a paper by Mr. T. C. Mendenhall, of Columbus, which appeared in the American Journal of Science, vol. ii., p. 157. This gentleman says: “An attempt was made to determine the relative rapidity with which responses are made to impressions made upon the different senses. . . . Time is measured on a register similar to the astronomical chronograph, in which I have been able to move a slip of paper with great regularity at the rate of about one and one-half inch per second, the seconds being registered upon the slip by a seconds pendulum according to the electric method. The person on whom the experiment is being made is seated at a table, having his hand on a key; by pressing this, the time of the action is registered on the paper. I made an apparatus, by means of which the circuit is completed for an instant the moment that there appears at a circular opening, about three-fourths of an inch in diameter, a card, red or white, as I choose, which completely fills the opening. The subject is instructed to watch this opening, and to press the key immediately on seeing the card. The actual appearance of the card and his closing the circuit in response are marked on the slip of paper by two dots about one-fifth of an inch apart (two-fifteenths of a second). This is a measure of the time occupied by the somewhat complex operation of his perceiving the object, and acting in response to that perception. I introduce the exercise of judgment by giving him two keys, one for each hand, and by instructing him that, when a white card appears, he is to close with his right hand, and when a red card appears, with his left. . . . According to the same general plan, I made trials concerning the sense of hearing. I arranged that, by