Bryn Mawr College monographs . aracter, thebroken circle (the international test object), which can beturned in different directions and the judgment of its directionrather than the recognition of the character be required of theobserver as a test of discrimination. Our reasons for thispreference are as follows: (i) A test letter may be recognizedwhen it is not seen at all clearly. Recognition is too de-pendent on extraocular functions to be used with precisionas a measure of ocular capacity. (2) The different letters ofthe Snellen chart set an unequal task for the resolving powerof the eye. (
Bryn Mawr College monographs . aracter, thebroken circle (the international test object), which can beturned in different directions and the judgment of its directionrather than the recognition of the character be required of theobserver as a test of discrimination. Our reasons for thispreference are as follows: (i) A test letter may be recognizedwhen it is not seen at all clearly. Recognition is too de-pendent on extraocular functions to be used with precisionas a measure of ocular capacity. (2) The different letters ofthe Snellen chart set an unequal task for the resolving powerof the eye. (3) An objective check s had on the is especially helpful in case of children, and the unin-telligent, untrained and subjective type of adult. (4) Bythe use of the same test character, turned in different direc- 64 C. E. FERREE AND GERTRUDE RAND tlons at will, all possibility of learning the test series iseliminated. Also the test-object becomes much more valu-able for the detection of astigmatisms. And (5) at low. 10 ao JO ^6 £0 FiG. lb. Calibration Chart. illuminations the eye fatigues very rapidly. Thus if thetask is the reading of the whole line of letters the resultsobtained measure not only acuity, but the power to sustainacuity which may or may not be compatible with the purposeof the test. As stated in the introduction, the apparatus was designedto meet a specific testing need of the Navy. However, it hasin addition the following laboratory and clinic uses, (i)Photopic acuity may be tested under the conditions of aconstant and uniform illumination of known intensity. Incase the test-object is a line or chart of letters, provision ismade that each letter receives, within sensible limits, equalamounts of light. (2) Scotopic or twilight vision may betested—also the amount and rate of scotopic precise and feasible means is thus afforded for testing thelight sense insofar as it affects the power to see clearly. (3) AN APPARATUS 65 If the image of
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