752 



PHYSIOLOGY, RECENT. 



the surface of the body in localizing impressions 

 made upon it. It may be objected, however, 

 that a comparison of this sort has little value, 

 inasmuch as it is the optical image of the object 

 which is applied to the retina, while the object 

 itself is brought in contact with the surface of 

 the body. It is interesting, therefore, to coin- 

 pare the absolute size of the smallest intervals 

 by which two external objects must be sepa- 

 rated in order that they may make distinct im- 

 pressions upon the two senses. In making the 

 comparison regard must be had to the distances 

 within which both senses can be practically 

 employed. The sense of touch can not be ex- 

 ercised beyond an arm's length, and the sense 

 of sight is useless for objects nearer than the 

 nearest point for which the eye can be accom- 

 modated. About thirty inches may be assumed 

 as the longest convenient range of touch, four 

 inches as the shortest of distinct vision. An 

 angle of 1', which has been taken as the mini- 

 mum visual angle, is subtended at the distance 

 of four and of thirty inches by lines of 0*03 and 

 0'2 inm. respectively. That is, two points, 0-03 

 mm. apart, at a distance of four inches from 

 the eye, are seen to be -distinct and separate, 

 and the same is true of points - 2 mm. apart 

 at thirty inches from the eye. Now, since, ac- 

 cording to AVeber, two points, in order to pro- 

 duce separate impressions upon the skin at the 

 ends of the fingers, must not be less than 2'2 

 mm. apart, it appears that within the limits of 

 four and thirty inches the sense of sight is from 

 ten to seventy times more accurate than that of 

 touch. In these tests, however, it is only the 

 accuracy and delicacy of two sensitive surfaces 

 that are compared together, but in common life 

 the data of sight and touch are intimately asso- 

 ciated with those of muscular sense; and the 

 inquiry needs to be continued to ascertain 

 whether the superiority of vision is main- 

 tained under these circumstances. The fact 

 that we depend more upon touch than upon 

 vision in estimating slight differences of level 

 in contiguous surfaces, as when we draw the 

 finger-nail over the line of junction, indicates 

 that this may not always be the case. 



The experiments of Drs. Bowditch and 

 Southard had especial reference to the de- 

 termination of the relative accuracy of sight 

 and touch, when aided by the muscular sense, 

 and were applied for the determination of the 

 precision with which a movement can be ex- 

 ecuted toward a point, the position of which 

 has been determined by sight or by touch. A 

 small object was put upon a white sheet of pa- 

 per in different positions. The experimenter, 

 having ascertained its position by sight, shut 

 his eyes and endeavored to touch the object 

 with the point of a pencil held in his right 

 hand. In another set of observations he him- 

 self put the object in position with the left 

 hand, having his eyes closed, and then, keep- 

 ing his eyes closed, endeavored to place the 

 pencil-point upon it as before. In a prelimi- 

 nary series of sixty trials, in half of which the 



position of the object was determined by sight, 

 and in the other half by touch, sight appeared 

 to be twice as accurate for the determination 

 as touch. The experiments were then varied 

 in different ways, employing indirect instead 

 of direct vision, to eliminate the aid that might 

 be given by the muscular sense of the muscles 

 that move the eye ; by using the same hand to 

 place the object and to search for it; to deter- 

 mine the effects of different intervals of time ; 

 and to ascertain the effect of maintaining the 

 head in a fixed position. In all, seven sets, of 

 six hundred experiments each, making in all 

 four thousand two hundred trials, were per- 

 formed. The result of the whole was, that 

 the most accurate spatial knowledge was ob- 

 tained by direct vision. The effect of fixing 

 the position of the head was to diminish the 

 accuracy of the localization, but even under 

 those circumstances the errors were, in nearly 

 every instance, less than those met with in the 

 other methods of experimenting. The method 

 next in accuracy was that of localization by 

 touch, with the same hand by which the move- 

 ment in search of the object was executed, the 

 head being free to move. " It may, at first 

 sight," observe the experimenters, "seem sur- 

 prising that this method should be less accu- 

 rate than that of direct vision. It would be 

 reasonable to suppose that the position of an 

 object having been once determined by the 

 sense of touch, it would be possible to place 

 the hand upon it a second time with greater 

 precision than would be possible when the 

 position had been determined by the sense of 

 sight. That the reverse is the case, is probably 

 to be accounted for by the fact that in our 

 daily life all our movements are guided by the 

 sense of sight to a much greater extent than 

 by the sense of touch." Next in order of ac- 

 curacy of localization came the experiments 

 with indirect vision ; next, those in which the 

 position of the object was determined by the 

 same hand with which the movement in search 

 of it was made, but the head was fixed. Least 

 accurate of all the methods employed was that 

 of touch with the opposite hand. The experi- 

 ments respecting the effects of time showed 

 that a slight interval is required for the forma- 

 tion of a mental image of the object in space, 

 and that this interval is about two seconds, to 

 which point the accuracy of the identifications 

 increased, and after which it diminished. The 

 results of the experiments, having been ob- 

 tained entirely from one person, are liable to 

 correction for the personal equation ; and it is 

 suggested that it would be a very interesting 

 extension of the research, to inquire how far 

 the various occupations of life affect the accura- 

 cy of execution of the movements. "It might, 

 perhaps, be expected that the power of the 

 blind to determine the position of objects by 

 the sense of touch would be found, when test- 

 ed in this way, to be much greater than that 

 of persons possessed of sight. A few prelimi- 

 nary experiments, however, which have al- 



