plus sign will move to the (right or left). Move this plus sign until it is exactly superimposed
on the plus sign of the central focusing circle. This completes the setting for the near
horizontal meridian.
The scale of the left-hand or horizontal measuring drum indicates the actual dioptric power
of the cornea in the horizontal or near horizontal meridian. The drum may be left at this
position during the rest of the procedure.
Measuring the Vertical meridian
When the horizontal meridian is found with an astigmatic cornea, the minus signs above
and below the centre circle will be doubled. They must now be made single by focusing the
instrument. This brings the vertical meridian into proper focus.
To measure the curvature in the vertical meridian, it is necessary only to superimpose the
short minus sign and the split minus sing by turning the right-hand measuring drum. The
scale on the right-hand drum then indicates the actual dioptric power of the cornea
curvature in the vertical or neat – vertical meridian. Remember that if the cornea is
astigmatic, it is impossible to get both principal meridians in focus at one time.
Note:
While measuring with either drum, the other hand should be constantly on the
focusing knob to keep the meridian being measured in sharp focus. This permits the
operator to keep the mire, whose images in the cornea he measures, a constant distance
form the eye in split of the small movements, which every eye constantly makes. Unless
the object distance is a fixed one, this measurement of the image size may be erroneous.
The operator should continuity take advantage of the critical focus capabilities provided by
the keratometer.
19
Corneal Astigmatism
On that left or horizontal measuring drum the power is established for the cornea in the
Summary of Contents for KMS 6
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