200 PSYCHOLOOT. 



Take again the contractions of the muscles which make 

 the eyeball rotate. The feeling of these is supposed by- 

 many writers to play the chief part in our perceptions of 

 extent. The space seen between two things means, accord- 

 ing to these authors, nothing but the amount of contraction 

 which is needed to carry ihe fovea from the first thing to the 

 second. But close the eyes and note the contractions in 

 themselves (even when coupled as they still are with the 

 delicate surface sensations of the eyeball rolling under the 

 lids), and we are surprised at finding how vague their space- 

 import appears. Shut the eyes and roll them, and you can 

 with no approach to accuracy tell the outer object which 

 shall first be seen when you open them again.* Moreover, it 

 our eye-muscle-contractions had much to do with giving us 

 our sense of seen extent, we ought to have a natural illusion 

 of which we find no trace. Since the feeling in the muscles 

 grows disproportionately intense as the eyeball is rolled 

 into an extreme eccentric position, all places on the extreme 

 margin of the field of view ought to appear farther from 

 the centre than they really are, for the fovea cannot get to 

 them without an amount of this feeling altogether in excess 

 of the amount of actual rotation. f When Ave turn to the 



* Volkmann, op. cit. p. 189. Compare also what Hering says of the in- 

 ability iu his owu case to make after-images seem to move when he rolls 

 his closed eyes iu their sockets ; and of the iusignilicance of his feelings of 

 convergence for the sense of distance (Beitriige zur Physiologic, 1861-2, 

 pp. 31, 141). Helmholtz also allows to the muscles of convergence a very 

 feeble share in producing our sense of the third dimension (Physiologische 

 Optik, 649-59). 



f Compare Lipps, Psychologische Studien (1885), p. 18, and the other 

 arguments given on pp. 12 to 27. The most plausible reasons for contrac- 

 tions of the eyeball-muscles being admitted as original contributors to the 

 perception of extent, are those of Wundt, Physiologische Psychologic, ir. 

 96-100. They are drawn from certain constant errors in our estimate of 

 lines and angles ; which, however, are susceptible, all of them, of different 

 interpretations (see some of them further on). — Just as my MS. goes 

 to the printer, Herr Mtlnsterberg's Beitriige zur experimentellen Psy- 

 chologic, Heft 3, comes into my hands with experiments on the measure- 

 ment of space recorded in it, which, in the author's view, prove the feeling 

 of muscular strain to be a principal factor in our vision of extent. As 

 Milnsterberg worked three hours a day for a year and a half at comparing 

 the length of lines, seen with his eyes in different positions ; and as he care- 

 fully averaged and ' percented ' 20.000 observations, his conclusion must be 

 listened to with great respect. Briefly it is this, that "our judgments of 



