November 30, 188:3.] 



SCIENCE. 



715 



a -waterfall or at a rotating-disk. Ilelmholtz, 

 indeed, explains all these appearances together 

 as visual vertigo ; putting them with the phe- 

 nomena of apparent motion in dizziness, and 

 regarding them as all alike caused by motions 

 of the eves, unconsciously continuing after the 

 cessation of the observation of the objective 

 motions. Yet Helmholtz has trouble to apply 

 this explanation, whose validity in its own 

 class of cases is undoubted, to the case where 

 contrary motions appear in the field of vision 

 at the same time ; and Ileriug, in Hermann's 

 'Handbuch der physiologic' (iii., i., nG'2) , in- 

 sists for these cases on the rival explanation, 

 " Die scheinbewegung beruht auf einer localen 

 reaction des schorganes gegen die vorangegan- 

 gene erregung." Thus we should have true 

 spectra of motion. 



One ma3- add, that the recent article by Drs. 

 H. P. Bowditch and G. Stanley Hall "in the 

 Journal of ])hysiology, vol. iii., p. 297 sqq., 

 leaves no room to doubt that optical illusions 

 of motion of this class do exist, that cannot be 

 explained as resulting from visual vertigo, and 

 that can properly be called beiveguyigsnachbil- 

 der, at least until we know more about them. 



If, now, the explanation of Helmholtz is not 

 suflScient for all cases, if there are any cases 

 of true bewegnngsnachbilder, then surely they 

 cannot be brought in any wise under Professor 

 Strieker's extreme theorj- without a simply 

 appalling mass of hypotheses. Such cases are 

 insisted upon by Fleischl in the note above 

 cited ; and he even notes the curioush' contra- 

 dictor3- character of the spectra of motion, — 

 the presence in them of a motion, without any 

 actual transferrence from place to place that the 

 eye can follow. They excite him to the rather 

 petulant outburst with which his note closes ; 

 viz., that empfindungen are fundamentalh" il- 

 logical, and that the principle of contradiction 

 does not bold good for them, but onl}- for their 

 more developed relatives, the vorstellungen. 

 Perhaps, however, our author will insist that 

 it was of vorstellungen only that bis studies 

 treat, and that with such wicked and illogical 

 empfindungen as Fleischl's bewegungsnachbil- 

 der he has notiiing to do. Yet, if his theory 

 is to be complete, he must not be allowed to 

 shrink from its applications. What can he do 

 with the own cousins of these illogical phe- 

 nomena, namely, the chaotic sensations of the 

 darkened visual field? Here is for some eyes, 

 such as the present reviewer's, little more than 

 motion or change, without any power of dis- 

 tinguishing what it is that moves. So it is 

 with Mr. Galton ('Human facultj-,' p. 159). 

 Helmholtz himself describes, in his own case. 



motions of ' two systems of circular waves ad- 

 vancing towards their centres ; ' and so, of 

 course, there must be for him. in the darkened 

 field, motions at the same time in contrary 

 directions, that cannot well be explained as the 

 result of muscular efforts. A similar experi- 

 ence is described by Professor LeConte (in his 

 book on 'Sight,' p. 72) ; and Purkinje's obser- 

 vations, as Helmholtz gives them, are also to 

 this effect. In all these cases, then, we have 

 motions — whether manifold and confused, or 

 definite and regular — which, it would surely 

 seem, cannot be explained as resulting from, 

 or in anj- wa3- implying, muscular sensations. 

 These cases, then, lie wholly out of Professor 

 Strieker's range. 



Yet possibly it may not seem to most readers 

 worth while to spend time in refuting the hasty 

 generalization of our author. But the object 

 here is to suggest both the necessary limitation 

 and the possible scope of this theory of the 

 ideas of motion. Its limited scope seems clear, 

 but its very one-sidedness is instructive if we 

 look a little closer. It is one-sided, for in- 

 stance, in the inductive methods used. In case 

 of the mental picture of the snow-storm. Pro- 

 fessor Strieker found his theory in danger of 

 failing : so he followed the single snow-flakes 

 with the mind's eye ; and lo ! the theory is veri- 

 fied, and so throughout. The influence of atten- 

 tion upon the result is so plain, that the reader 

 must have noticed the fact in reading our pre- 

 vious summary of the book ; and yet this for- 

 mal error in the reasoning does not make the 

 result whollv erroneous. If one takes note in 

 himself of the facts upon which such stress is 

 laid b^- our author, one will ver^' readilv find 

 that there is at least this in them ; viz., every 

 clearh' conceived or perceived objective motion 

 tends, just in proportion to the clearness and 

 defniteness of perception or of conception, to 

 become associated with a certain kind, degree, 

 and direction, of muscular effort. That muscu- 

 lar eflbrts are involved in mapping out the vis- 

 ual field ; that we follow ever}- point in whose 

 motion we take special interest, and are par- 

 tially conscious of what we do in following it ; 

 and that analogous facts exist for tiie sense of 

 touch, — are truths now generally recognized. 

 Professor Strieker is interesting as having given 

 us an independent, and, in so far forth, un- 

 prejudiced, contribution to the theory. That it 

 has charmed him over-much is itself a fact of 

 interest for the theory : for it shows how much 

 clearer and better Professor Strieker seemed 

 to himself to have conceived motions, when 

 he had brought their conception into immedi- 

 ate connection with the facts of the muscular 



