HELMHOLTZ IN HEIDELBERG 



the will, at least up to a certain point, but it was 

 reserved to Helmholtz to show that there are other 

 and slighter movements that are altogether involun- 

 tary. Thus no one can voluntarily diverge the visual 

 lines ; in other words, it is impossible voluntarily to 

 cause simultaneous contraction of both external recti. 

 Nor can we voluntarily rotate the eyeball round the 

 antero-posterior axis, but here, again, slight involuntary 

 movements may be made. We can thus turn the line 

 of vision into every possible direction, but when its 

 direction has been fixed, the position of the eye is also 

 fixed, and is beyond our control. 



Helmholtz studied the subject by taking advantage 

 of the method of Donders, which, by an ingenious 

 device, he greatly improved. In this method, the 

 apparent position of after-images produced by exhaust- 

 ing the retina, say, with a red or green object, was 

 compared with that of a line or fixed point gazed 

 at with a new position of the eyeball. The ocular 

 spectra soon vanish in this experiment, but a good 

 observer can determine with great accuracy the coin- 

 cidence of lines with the ocular spectra. Thus, after 

 producing an after-image with the head in the erect 

 position, the head may be placed into any inclined 

 position, and if the attention is then fixed :on vertical 

 lines, it can easily be seen whether the after-image 

 coincides with the lines. As the after-image must 

 remain in the same position on the retina, if it coin- 

 cides with the vertical lines, it is evident there must 



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