Perhaps the most widely used test of human powers is that of acuity of vision. In proportion as it is widely used there is need to provide a good and as far as possible a foolproof equipment. As the situation now is, the testing of vision is one of the most loosely conducted tests. Not only has there been no substantial change in the principles, test materials and procedure of making the test since the days of Snellen, but even the principles laid down by him are not complied with in a great part of the testing that is now being done. So far as restriction or supervision is concerned, almost any type of test chart may be used under any type or intensity of illumination. The variable difficulty of the task to which the eye may be subjected under these conditions of testing is a sufficient guarantee that consistency
FERREE CE, RAND G. A MORE NEARLY ABSOLUTE METHOD OF TESTING AND RATING VISION. Arch Ophthalmol. 1940;24(2):292–315. doi:10.1001/archopht.1940.00870020084009
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