714 



SCIENCE. 



[Vol. II., No. 43. 



Professor Strieker asserts that practice in 

 the use of his muscles, and especially' in the 

 training of the muscular sense for mechanical 

 purposes, has rendered ■ him uncomnionh' well 

 qualified to note the presence of muscular sen- 

 sations as elements in any complex state of 

 mind. Some of his colleagues have like skill. 

 He has thus been led to paj- attention to facts 

 such as, that, when he perceives the movements 

 of another person, or remembers these move- 

 ments clearly afterwards, or deliberately im- 

 agines a movement of a man or even of an 

 animal, he always is aware of a slight feeling 

 of effort in those muscles of his own body that 

 would be concerned in the same or in some 

 analogous movement. The appreciation or 

 conception of a bodily movement is thns 

 accompanied 'by a more or less well-marked 

 dramatic imitation of the movement. Again : 

 if he perceives or conceives the visible motion 

 of a body in space, he is conscious of a motion, 

 or of a tendenc}'' to motion, in the muscles of 

 the eye. These personal observations he finds 

 confirmed bj' others in proportion to their 

 training in introspection, and in the special 

 observation of the muscular sensations. In 

 watching the motions of manj? small objects at 

 once in the field of vision, as in case of a 

 snow-storm, the author is not quite so fortu- 

 nate. "I find difficult}-," he saj-s, "in dis- 

 covering any trace of motions of the eyes ; j'et, 

 after long exercise, I have now no longer the 

 least doubt that I follow the single flakes with 

 small and quick motions or nascent motions of 

 the ej'es " (p. 23). In case, however, of an 

 effort to picture in memory just how a snow- 

 storm looks, the author either finds himself 

 picturing a stationary' mass of flakes, or else 

 following in mind the motions of single flakes. 

 In the latter case he discovers that the muscles 

 of the e3"es are perceptibl}' innervated. The 

 result, therefore, notwithstanding the difficult}', 

 is in the end the same. 



In the case of the illusions of motion in 

 the ' wheel of life,' the author asserts that the 

 illusion is alwaj-s accompanied by motions of 

 the eyes, and tliat it is impossible without such 

 motions. 



His conclusion from all this is, that " motion 

 is conceivable only in connection with, and by 

 means of, the muscular sense," — a result that, 

 in this extreme form, probably x&ry few investi- 

 gators will accept. Certainly Professor Striek- 

 er has not proved it ; since he has, on the one 

 hand, left verj' numerous facts wholly un- 

 noticed, and, on the other hand, has adduced 

 facts that are of doubtful force for his purpose. 

 As for the omitted facts, a reviewer of this book 



in the PhilosopJiische monatshefte has chal- 

 lenged Professor Strieker to show what part 

 the muscular sense plays in the perception of 

 the motion of an object seen double in indirect 

 vision, when the e3-es are fixed on some chosen 

 point. Thus, if one's gaze is fixed direcll}* in 

 front on some bright point, or on one of the 

 eyes of the observer's own face as seen in a 

 mirror, so that the eyes are surely at rest, then 

 the finger, or a pencil, held up so as to appear 

 double, will v'ct in both its shadow}- images be 

 seen to move when the real finger is moved, 

 or when the pencil is moved by an assistant 

 without the observer's previous knowledge. 

 Yet here, says the reviewer, the double images 

 show that the eye does not follow the motion 

 at all, else they would coalesce. And if the 

 mirror is used, the observer, looking at his own 

 eye in the mirror, can be doubly sure that his 

 eyes are motionless. This objection, however, 

 is not so near at hand as another, mentioned 

 by the same reviewer, — the one that must at 

 once occur to any reader of Professor Strieker's 

 book; viz., the case of the motion of some 

 small object over the skin, say a crawling in- 

 sect. Here the motion is felt as motion, and 

 not as mere tickling, as soon as the requisite 

 speed and amplitude are attained. What has. 

 the muscular sense to do here? 



But, obvious as these objections are, they are 

 not final. Professor Strieker might reply, that, 

 according to Lotze's own suggestion, the now 

 well-recognized localzeichen themselves may 

 be of the nature of muscular impulses. In the 

 retinal field the tendency to bring any point of 

 attention into the point of sight may exist 

 universally ; and the motion of the indirectly 

 seen finger over the resting retinal field may 

 be known by reason of the change in the magni- 

 tude and direction of the effort that during 

 the experiment constantly exists, to bring the 

 finger, as the object most attended to, into 

 the point of sight. Something analogous may 

 make possible the perception of the motion 

 of a point on the skin. But these are hypo- 

 theses. They are doubtful ; and they require 

 of Professor Strieker supplementary investi- 

 gations, whereof he seems to have had no 

 thought. 



There remain, however, the cases of what 

 a late writer in the Wienei' sitzungsherichte 

 (Fleischl, Optisck-physiol. notizen, no. vi., 

 in bd. Ixxxvi., i., for 1882) has called bewe- 

 gungsnachbikler, which have long been ob- 

 served and discussed. These are the subjective 

 appearances of motion in the visual field, after 

 the continued observation of swiflly-moving 

 real objects ; as when one has been looking at 



