The use of the illumination scale for the detection of small errors in refraction and in their correction.

There are doubtless many ways in which sensitivity can be added to the acuity test for the detection of small errors in refraction and in their correction.In connection with the problems which we have undertaken during the past eight years involving modifications and refinements in functional testing, three principles have come to light which can be used very effectively to this end.That is, the eye which suffers from an insufficient resolving power shows the following functional defects.(1) An undue lag or slowness of discrimination and of making the adjustments needed for clear seeing.(2) A marked loss in power to sustain the adjustments needed for clear seeing.And (3) an increase in the amount of light required just to discriminate details in the standard acuity object.The devising of test methods based on the first two of these principles has been treated of in former papers.The third alone will be considered here.The relation of the illumination scale to the detection of small errors in refraction and in their correction may be stated briefly as follows.Insofar as the test-object is concerned, clearness of seeing depends upon the value of the visual angle subtended, and the intensity of the illumination.It follows from this that either the illumination scale or the visual angle scale may be used for the detection of errors in refraction, i.e., in the diagnostic procedure either the illumina-243