190 Prof. Relmholtz— Normal Motions of the [April 14, 



dian plane of the eye. Such a plane cuts through the retina in a certain 

 line ; and when the eye has been moved, we consider as the same meridian 

 plane that plane which passes through the new direction of the visual 

 line and the same points of the retina as before. 



After having given these definitions, we may express the law of the 

 motions of the eye in the following way : — 



Whenever the eye is brought into a secondary position^ that meridian 

 plane of the eye which goes through the primary directiort of the visual 

 line has the same position as it has in the primary direction of the eye. 



It follows from this law that the secondary position of the eye may be 

 found also by turning the eye from its primary position round a fixed axis 

 which is normal as well to the primary as to the secondary of the visual 

 line. 



[The geometrical relations of these different positions were explained by 

 the lecturer by means of a moveable globe placed on an axis like the common 

 terrestrial globes.] 



It would take too long to explain the different ways in which dif- 

 ferent observers have tried to determine the law of the motions of the 

 eyeball. They have employed complicated apparatus for determining the 

 angles by which the direction and the rotation of the eye were to be 

 measured. But usually two difficulties arise from the use of such instru- 

 ments containing graduated circles, in the centre of which the eye must be 

 kept steady. In the first place, it is very difficult to fix the head of the 

 observer so firmly that he cannot alter its position during a continuous 

 series of observations, and that he reassumes exactly the same position of 

 the head when he returns to his measurements after a pause, — conditions 

 which must necessarily be fulfilled if the observations are to agree with 

 each other. Secondly, I have found that the eye must not be kept too 

 long a time in a direction which is near to the limits of the field of vision ; 

 else its muscles are fatigued, and the positions of the eyeball corresponding 

 to different directions of the visual line are somewhat altered. But if we 

 have to measure angles on graduated circles, it is difficult to avoid keeping 

 the eye too long in directions deviating far from the primary direction. 



I think that it depended upon these causes, that the observations carried 

 out by Meissner, Tick, and Wundt agreed very ill with each other and 

 with the law which I have explained above, and which was first stated by 

 Professor Listing of Gottingen, but without any experimental proof. 

 Happily it is possible, as I found out, to prove the validity of this law by 

 a very simple method, which is not subject to those*sources of error I have 

 named, and which I may be allowed to explain briefly. 



In order to steady the attitude of the head in reference to the direction 

 of the visual line, I have taken a little wooden board, one end of which is 

 hollowed into a curve fitting the arch of the human teeth ; the margin of 

 this hollow is covered with sealing-wax, into which, after it had been 

 softened by heat and had been cooled again sufficiently, I inserted both 



