THE CENTRE OF MOTION OF THE EYE. 417 



at the prism, and then at the mirror. A vertical line is drawn with black 

 sealingwax on the surface of the prism next the eye, and the board is moved 

 towards or from the eye till this line appears as a tangent to the front of 

 the cornea, while the dot still is seen to cover the centre of the image of 

 the pupil. The only way in which the position of the board can now vary 

 with respect to the eye is by turning round the line of vision as an axis, 

 and this is prevented by the board being laid on a horizontal platform carried 

 by the teeth. 



If now the eye is brought into two different positions and the board 

 moved on the platform, so as to be always in the same position relative to 

 the eye, we have to find the centre about which the board might have 

 turned so as to get from one position to the other. 



For this purpose two holes are made in the platform, and a needle thrust 

 through the holes is made to prick a card fastened to the upper board. We 

 thus obtain two pairs of points, AB for the first position, and ab for the 

 second. 



The ordinary rule for determining the centre of motion is to draw lines 

 bisecting Aa and Bb at right angles. The intersection of these is the centre 

 of motion. This construction fails when the centre of motion is in or near 

 the line AB, for then the two lines coincide. In this case we may produce 

 AB and ab till they meet, and draw a line bisecting the angle externally. 

 This line will pass through the centre of motion as well as the other two, 

 and when they coincide it intersects them at right angles. 



VOL. II. 



53 



