7 
to be contained in a ribbon approximately 1 minute of arc wide around 
the edge of the target. The most precise pointings were made by 
selecting a measuring mark to give a minimum annulus within this 
ribbon irrespective of the target size in the range examined. 
To complete the study of pass-point measurement, the investi 
gation should be extended to consider the effects of contrast and 
background detail. The background detail may be significant in its 
orientation, and in its position with respect to the retinal re 
ceptive fields. 
The earlier stereocomparator tests demonstrated the precision 
which could be expected in operational measurements using existing 
instruments and method^. The investigations described in this paper 
extend this approach towards the ultimate precision attainable with 
the eye, to demonstrate the need for a compatibility analysis of all 
the elements in a measuring system if maximum efficiency in design 
is to be obtained.