396 THE POPULAR SCIENCE MONTHLY. 



the star, so as to excite the nerves of vision, are both very small : it is 

 the coordinating power of the brain that works slowly aucl absolute 

 personal equation is largely the measure of the time required for the 

 brain to superpose two different sensations, to coordinate 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 observ- 

 ing 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 equa- 

 tion remained almost constant (see table) : 



j The second marked by sound, : e = + S .10 80 observations. 



I " " " " sight, : e= + 0.08 80 " 



j The second marked by sound, : e = + s . 11 80 " 



( " " " " feeling, :e= + 0.11 80 



This table seems to bear out M. "Wolf's view ; but, in this connec- 

 tion, 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 w r atch this opening, and to press the key im- 

 mediately 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 



