780 
Fig. 2 gives an example of the more general case that these 
points do not coincide. 
The investigation is to be separated into two parts. In the first 
part we shall describe the investigation into the direction of the 
line of regard, whilst in the second part the investigation of the 
rotation of the eye round its line of regard is treated. 
The purpose of the first part of the investigation is to find the 
point 7, where the line of regard of the examined eye cuts the 
wall, whilst the other eye fixes the point Q. 
For this purpose we place exactly for ( a luminous point 
(short candle-flame) and place before the examined eve the rods 
of Mappox in a trial-frame. As long as the investigation lasts 
the fixing eye must constantly look at the point Q. Now the rods 
of Mappox in the trial-frame are turned till the luminous red line 
that the examined eve observes, apparently goes through Q. 
We know then that the red line on the retina goes through the 
fovea of the examined eye. Now we remove the flame from Q and 
move it in a cirele round (Q, whilst the rods of Mappox before the 
examined eye have remained unaltered in their places. Twice 
the patient will then observe that the red line goes apparently 
through (. The points where the flame is at these moments ((Q’ 
and Q") are likewise marked on the wall. We know that we can 
draw straight lines through the points Q, Q', and Q". which will 
likewise go through the point P, on which the line of regard of 
the eye behind the rods is directed, which we want to find. As on 
account of the unaltered position of the rods, the observed red line 
can only move parallel to itself, these three lines which contain all 
