DEAR DOCTOR: Your note asking my opinion of the comparative
usefulness of the Maddox rod and the phorometer is at hand.
In the detection of heterophoria I regard the rod as the most
convenient and trustworthy instrument used.
The distance at which the test is made and the dissimilarity of the
images seen usually eliminate all actual effort to hold the eyes in
any particular position other than that in which they stand the most
easily. Accordingly the deviation is quickly noted and readily
measured.
So satisfactory has this modest little instrument been in my
examinations that I now rarely resort to other methods. The amount
of deviation sometimes shown between this and other instruments is
so slight as to make little or no difference in the measures
employed for correction.
It is to be noted that cases not unfrequently occur in which a
hyper-sensitive, or, on the contrary, an enervated condition exists,
which is not fully indicated by any instrument. An educated judgment
will here have to supply conclusions not to be drawn by any
hard-and-fast rules.
After the rod and the phorometer came into use and an opportunity
was presented to compare the results obtained by each, I made a
careful test of eighty pronounced cases of errors of refraction
accompanied by heterophoria. Of this number only nine showed a
persistent difference of deviation and in none of them a difference
greater than 1½°. But this was not always on the one side or the
other, as six out of the nine showed a higher degree of deviation by
the rod than by the phorometer. Eighty cases may not be enough upon
which to base an orthodox conclusion; but my experience with the rod
has been so satisfactory that I now seldom use the phorometer at
all. It appears quite possible practically to estimate the degree of
heterophoria as accurately with the one instrument as with the
other; and while it is true that a correction of the error of
refraction will commonly correct the deviation, still all cases of
optical defect should be tested with the rod or phorometer before
the lenses are prescribed.