442 FERREE AND RAND— LIGHTING 



tional efficiency of the eye is measured by its power to sustain clear 

 seeing for a period of time. In operation the test may be described 

 briefly as follows: The power of the eye to sustain a certain standard 

 of acuity for three minutes is measured before and after a 3-hour 

 period of reading from uniform type and paper under the lighting 

 conditions to be tested. That is, by means of a visual acuity test 

 object, with the proper auxiliary apparatus for its control and obser- 

 vation, and a kymograph and chronograph, records are made of 

 the time the eye can be held up to this standard of performance and 

 the time it drops below. The ratio of these quantities to each other, 

 or to the total time for which the record is made, is taken as the 

 measure of the ability of the eye to sustain its power of clear seeing 

 before and after work under the lighting conditions to be tested. 



Thus far the analytical tests have been confined to the retina 

 and the extrinsic muscles of the eye. There are four ways in which 

 the retina might be expected to show a depression of functional 

 power: in a lowering of sensitivity to colored and white light; in an 

 increase in the rate of exhaustion to light stimulation and a corre- 

 sponding decrease in rate of recovery ; and in an increase in the lag 

 or time required to give its full response to light stimulation. We 

 have already made tests for the first three of these features for the 

 effect of different lighting conditions and work is under way for the 

 testing of the fourth feature. In the work on the extrinsic muscles 

 we have again found it advisable for the sake of sensitivity in detect- 

 ing small effects to use an endurance test instead of one requiring 

 only a momentary performance. That is, we have supplemented the 

 conventional abduction and adduction tests by a determination of the 

 power to sustain the coordination of action on the part of these 

 muscles needed for binocular seeing — measured by the power to 

 maintain under strain the accurate combination of binocular images 

 of a simple test-object before and after a period of work under the 

 lighting conditions to be tested. The eyes are put under strain to 

 combine their images to give the needed sensitivity to the test. 

 When this is done even when the muscles are fresh, if the object is 

 looked at or fixated for an interval of time, it will be seen alternately 

 as one or as two. The proportion or ratio of the time seen as one 

 to the time seen as two or to the total time of the observation can 



