42 
sponding to the line in question. Thus the uppermost line 
should be recognised at 60, the second line at 36, the 
third at 24, the fourth at 18, the fifth at 12, the sixth at 9, 
and the seventh at 6 metres. 
Directions for itsing the E Test- Type, 
1. Measure off 6 metres. 
2. Let the test-card be hung or held upright, in a good 
light, at this distance in front of the person to be examined. 
3. He is now instructed to indicate as explained above, 
the different positions of the E, beginning at the top of the 
card and proceeding downwards as far as he can. 
Note. —If he distinguishes accurately the lowest line , 
the trial-distance is increased until we find the maximum at 
which this line can be seen with precision : it may thus be 
necessary to greatly increase the distance in exceptional 
cases. To avoid an obvious fallacy in such circumstances, 
it is essential to have several copies of the lowest line (6), 
differing in the arrangement of the type. 
If, on the other hand, he cannot distinguish the upper¬ 
most line at six metres, the trial distance is diminished until 
he can just succeed in doing so. 
4. The usual acuity in each case is indicated by a fraction, 
the numerator of which is the trial-distance in metres, and 
the denominator the figure on the card corresponding to the 
lowest line distinguished. 
Average vision will thus be expressed by f (or 1). 
Examples .—If at 6 metres the man can distinguish the 
line indicated on the card as “ 12,” but not the next lower 
line, his visual acuity = T \, i.e ., half average vision. 
If he cannot distinguish the uppermost line (“60”) at 
more than 2 metres, his acuity = i.e., one-thirtieth of 
average vision. 
If he can distinguish the lowest line (“6”) at 18 metres, 
his visual acuity = or three times the accepted average. 
In the employment of any method of testing vision, each 
