LIGHTING IN ITS RELATION TO THE EYE. 441 



which the eye is able to discriminate; the other, a principle equally 

 old, is that a loss of efficiency in a machine, apparatus, or a living 

 organ or organism will show out more plainly when a prolonged 

 rather than a momentary performance is required. These principles 

 in their simplest terms have been combined into a test of the com- 

 parative ability of the eye to maintain its power of clear seeing or 

 aggregate functional activity under different conditions of lighting 

 and under different kinds and conditions of use. Such a test for 

 clear seeing was needed because the conventional acuity test had not 

 been found to be sufficiently sensitive to fatigue conditions to warrant 

 adoption in our work. It, we scarcely need to point out, was designed 

 to test the dioptric condition of the eye and may be used with more or 

 less success as a test of how far a given lighting condition is con- 

 ducive to clear seeing with a maximum of momentary effort ; but it 

 has not the essentials of a fatigue test, nor of its converse, the ease 

 with which clearness of seeing is maintained, which are the features 

 needed primarily for the selection of lighting conditions for the 

 greater part of the work that we are ordinarily called upon to do. 

 Almost, if not quite, as good results, for example, may be gotten 

 with it after work as before when there is every other reason to 

 believe that the eye has suffered considerable depression in func- 

 tional power. The reason for this is obvious. Although greatly 

 fa*^igued, the eye can under the spur of the test he whipped up to 

 give almost if not quite as good results as the non-fatigued organ 

 when only a momentary effort is required. If fatigued, however, 

 it can not be expected to maintain this extra effort for a period of 

 time. The demonstration of this fact led early in our work to the 

 introduction of a time element into the test. The principle involved 

 is not a new one. It is merely the application of a very old and well- 

 known one to the work of testing for ocular fatigue. If, for ex- 

 ample, a sensitive test is wanted for the detection of fatigue in a 

 muscle, as good results can not be expected if the test requires only 

 a momentary effort on the part of the muscle as would be attained 

 if the endurance of the muscle were taken into account. For our 

 purpose, therefore, the old acuity test subjected to certain features 

 of standardization for the sake of greater reproducibility has been 

 made into an endurance test in which the fatigue or loss of func- 



