FUSION OF MUSCULAR WITH OTHER SENSATIONS. 1025 



senses gain spatial quality measurable in terms of the intensities of 

 the muscular sensations. 



Concomitant muscular sensations have been regarded as endowing sen- 

 sations of other modality with spatial quality. Lotze developed his scheme 

 of " local signs " 1 from the supposition that visual space-perception rests on 

 measurement of retinal differences in terms of intensities of muscular sensations 

 afforded by muscular movements required, when the various retinal points 

 are replaced by the fovea for purposes of attentional vision. Miinsterberg 2 

 goes so far as to write, " Every content of consciousness requires a muscular 

 impression " ; and it has been said " only when we move a limb do we become 

 conscious of its existence." 3 



A longer line can be visually just distinguished to be longer than a 

 shorter when the difference amounts to -$ of the length of the latter. 

 If we look through a horizontal slit toward a white screen, and without 

 being able to see any other objects perceive a vertical thread, each eye 

 will of itself take up such a position that the image of the thread falls 

 on the yellow spot. The visual axes intersect at the thread. On bring- 

 ing the thread nearer or removing it farther from the eyes, the angle of 

 intersection of the visual axes is changed as the eyes follow the thread. 

 If the thread is moved very little, the rotation of the eyes is so slight 

 that the accompanying movement sensation is not noticeable. It is 

 found that the increment of movement, which just suffices to cause 

 noticeable sensation, amounts to approximately -J^- of the total excursion 

 counted from the parallel position as zero. That perception of spatial 

 magnitude is thus directly proportional to the liminal difference of 

 sensation of movement possessed by the eye, suggests that the sensation 

 of movement serves as criterion for the space-perception. 



Again, we possess the finest sensitivity of our eyeball movements 

 when the visual axes are practically parallel, a rotation of about 1' 

 is then noticeable. If we suspend two black threads near together in 

 front of a white screen, and fixate them with one eye, and then retire 

 gradually from them, though keeping them constantly fixated, the 

 apparent distance between the threads constantly lessens as the retinal 

 image grows smaller. At a certain distance the two threads appear 

 one. There is a certain magnitude of the retinal image of two points 

 (i.e. a certain magnitude of the visual angle between them) below which 

 they cannot be perceived as separate. The two images fuse to one at 

 the moment when the distance between their retinal images is so small 

 that the eye has to turn only about 1' in order to bring first one 

 thread and then the other on the same retinal point. The limit to 

 which the eye can attain in cognition of spatial distance is identical 

 with the limit of its apprehension of the sensation of its own movement. 



These are quoted as instances of the grounds upon which the fusion 

 of muscular sensations of various intensity with perceptions of certain 

 other senses is regarded as endowing the latter with the quality of 

 " space." 



1 "Med. Psychol.," 1822, S. 331, etc. 



2 "Beitr. z. exper. Psychol.," 1890. 



3 George, "Lehrbuch der Psychol.," 1854, S. 231. 



VOL. II. 65 



