366 
centre of the pupil, the normal to the mirror through the dot 
is the visual axis of the eye—a determinate line. 
A right-angled prism is fixed to the board near the eye in 
such a position that the eye sees the image of its own cornea 
in profile by reflexion, first at the prism, and, then at the mir- 
ror. A vertical line is drawn with black sealingwax on the 
surface of the prism next the eye, and the board is moved to- 
wards 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 inter- 
section 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. 
