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However, this latter ignored the dynamics of accommodation. The under-corrected patient was more

likely to accommodate to place the circle of least confusion upon the retina. a was usually found in cases

of moderate hyperopic astigmia. Hence, Copeland recommended that the C.C. test be performed with

the patient slightly in the minus. or just a step more than the criterion set by Pascal. Copelands

recommendation permitted the patient to choose the spherical equivalent of whatever sphero-

cylindrical combination may have been determined before the C.C. test was applied. By either using the

correction C.C. of Pascal or adding the appropriate sphere with each cylinder, the status was maintained

during the test by the patient by utilization of his accommodation.

6. C. C. and Line Chart (Reversal Test) (Pascal. 1 952a)

a. The astigmatic power may also be tested upon a line cross astigmatic dial, astigmatic T, or similar

chart affording lines in the two principal meridians. The chart is presented so that one line of the cross

or T is on the assumed axis. In contrast to the test with letters, the patient should be slightly fogged. The

C.C. is placed as in the test for power with letters, and the relative blackness of each of the lines of the

cross on the chart compared as the C.C. is flipped. If the power is correct, the line coinciding with the

minus axis of the C.C. will appear blacker in each position. If the power is incorrect, the lines will appear

approximately equal in one position, reversed from the above, or the extent of blackness will not be the

same in both C.C. positions. Minus cylinder, with its axis coinciding with the minus axis of the C.C. when

in the position where the coinciding line is not as black is added to the correction.

(1) Vhereas in testing with letters, Pascal recommended that the power of the C.C. should approximate

that of the correction cylinder, in this test it was recommended that a weak C.C. should be used, not

exceeding a 0.25 IX combination with a 0.12 sphere combined with a + 0.25 cylinder preferable.

b. The axis of the cylinder may also be checked by the C.C. and a line of a T chart (Crisp-Stine Test). A

chart containing a cross with a marker at the intermediate position is aligned so that the markercoincides with the cylinder axis. The cross cylinder is placed as in the usual C.C. test for axis and rotated

about this marker. The two lines composing the cross are compared for relative blackness. If the lines

are not equal, the correction cylinder is moved towards the position of the cross cylinder axis which

coincides with the nature of the power of the correction cylinder (a minus cylinder towards the minus

axis. of the cross cylinder or a plus cylinder towards the plus axis of the cross cylinder). For each move of

the correction cylinder, the target chart is repositioned to the same degree. The Lebensohn arrow chart

may also be used for this test. This is continued until both lines appear approximately alike.

(1) 0.04 D. of cylinder difference is manifested by a .25 IX cylinder placed 50 off axis, but patients are

unable to tell the difference. The patient may call one of the lines doubled instead of blurred.

7. Differences Between the Cross cylinder Technique And Others

a. In many instances, a variation is found between the cylinder determined by line chart or clock dial

methods by rotating the cylinder and by the C.C. In the latter two, although the optics of both are

concerned with combinations of oblique cylinders, it must be noted that one depends upon a sudden

contrast of two blurs at approximately equal positions of arc, while the other depends upon a

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comparison of the best vision at one position with worsening vision, gradually introduced, up to a limit

in either direction (Pascal, 1952). Some patients may be able to make a more accurate judgment in one

procedure than the other, although the consensus seems to indicate that the sudden contrast (C.C.) is

generally easier for determination. Likewise, the clock dial procedure depends upon the sensitivity and