476 FERREE AND RAND— LIGHTING 



select only those who had already shown a satisfactory degree of 

 precision in other work in physiological optics and whose clinic 

 record showed no uncorrected defects of consequence. All were 

 under thirty years of age. Before being allowed to take part in the 

 actual work the results of which were used in the comparative study, 

 each observer was trained to a satisfactory degree of precision in a 

 preliminary series of tests with the light exposed at several points 

 in the field of view. In the actual work of testing the results were 

 compiled from a number of observations and the precision was 

 checked up by the size of the mean variation. No results were ac- 

 cepted as significant unless the variation produced by changing the 

 position of the source in the field of view was largely in excess of 

 the mean variation or mean error for each position tested. When an 

 exposure was to be made, the fixation was taken, the light turned on 

 and a record was made by the observer on a kymograph of the time 

 that was required for just noticeable discomfort to be set up; or, if 

 it were desired, when the dififerent stages of discomfort were reached. 

 The judgment was found to present no special difficulty and the 

 method, when properly applied, to provide a feasible means for com- 

 paring the sensitivity of the eye to discomfort under all the condi- 

 tions to which we have been able thus far to extend its application. 

 In actual practice the method also brings out an analysis of ocular 

 discomfort. 



Ocular discomfort seems to be a complex of three experiences 

 each of which develops at a different time. When the light is turned 

 on, we have at once glare. This is a light sensation and though un- 

 pleasant has no painful elements. Next comes a conjunctival sen- 

 sation which begins with what is commonly called " sandiness " and 

 soon passed over into a sharp, stinging, stabbing pain. Lastly there is 

 what is probably a muscular discomfort, a hurting and aching in the 

 ball of the eye which, if the exposure is continued long enough, seems 

 to radiate to the socket and the surrounding regions of the face and 

 head, the arch of the brow, the forehead, the temples, etc. 



The comparative determinations were made in 12 meridians of 

 the field of view from the center to the limit of vision in the given 

 meridian at points separated by 15 degrees. Space will be taken here 

 only for a general statement of result. In general for all of our ob- 



