192 Prof. Helmholtz— iVoma/ Motions of the [April 14, 



fixed, and that the eye, which wants only to glance for an instant at a 

 peripheral point of the drawing, and then goes back again to the centre, 

 is not fatigued. 



This method of finding the primary position of the eye proves at the 

 same time that vertical and horizontal lines keep their vertical or hori- 

 zontal position in the field of vision when the eye is moved from its pri- 

 mary direction vertically or horizontally ; and you see, therefore, that 

 these movements agree with the law which I have enunciated. That is 

 to say, during vertical movements of the eye the vertical meridian plane 

 keeps its vertical position, and during horizontal movements the horizontal 

 meridian. 



Now you need only bring either your own head into an inclined position, 

 or the diagram with the lines, and repeat the experiment, putting your 

 head at first into such a position that the centre of the diagram corre- 

 sponds with the primary direction of the visual line, and moving after- 

 wards the eye along the lines ab or cd/m either a parallel or perpendicular 

 direction to the coloured line of the diagram, and you will find the ocular 

 spectrum of the coloured line coinciding with those black lines which 

 are parallel with ab. In this way, therefore, you can easily prove the 

 law of Listing for every possible direction of the visual line. 



I found the results of these experiments in complete agreement with 

 the law of Listing for my own eyes, and for those of several other persons 

 with normal power of vision. The eyes of very short-sighted persons, on 

 the contrary, often show irregularities, which may be caused by the elon- 

 gation of the posterior part of those eyes. 



These motions of our eyes are a peculiar instance of motions which, 

 being quite voluntary, and produced by the action of our will, are never- 

 theless limited as regards their extent and their combinations. We find 

 similar limitations of motion of the eyes in other cases also. "We cannot 

 turn one eye up, the other down ; we cannot move both eyes at the same 

 time to the outer angle ; we are obliged to combine always a certain de- 

 gree of accommodation of the eyes to distance, with a certain angle of 

 convergence of their axes. In these latter cases it can be proved that 

 the faculty of producing these motions is given to our will, although our 

 will is commonly not capable of using this faculty. We have come by 

 experience to move our eyes with great dexterity and readiness, so that 

 we see any visible object at the same time single and as accurately as 

 possible ; this is the only end which we have learnt to reach by muscular 

 exertion ; but we have not learnt to bring our eyes into any given position. 

 In order to move them to the right, we must look to an object situated 

 on our right side, or imagine such an object and search for it with our 

 eyes. We can move them both inwards, but only when we strive to look 

 at the back of our nose, or at an imaginary object situated near that 

 place. But commonly there is no object which could be seen single by 

 turning one eye upwards, the other downwards, or both of them out- 



