Targets comprising a horizontal black line 5' thick offset by 1' were printed on strips to stick to the drums of the Catford Visual Acuity Apparatus. The 'vernier' targets so produced corresponded to the different sizes of Snellen letters from 6/6 to 2/60 when presented at 50 cm. Twenty-five children between the ages of 3 and 12 years with known amblyopia (mostly strabismic) were tested in a double blind trial to compare the visibility of the vernier targets with subjective Snellen acuity. All vernier results, both for better and amblyopic eye, were within one line of Snellen acuity. One hundred and twenty normal preverbal infants with normal eyes under the age of 2 1/2 years were tested with the vernier strips. Normal infants under the age of 6 months recorded a vision of 6/60-2/60; normal infants aged 6 to 12 months recorded 6/24-6/36, and infants between the ages of 1 and 2 1/2 years recorded 6/12-6/18.