IN ITS RELATION TO THE EYE. 475 



of surface brilliancy in the field of view and many variations of in- 

 tensity and color value. Of these variations the results of only one 

 will be given here, namely, the exposure of the brilliant surface at 

 different points in the field of view for one eye when fixation and 

 accommodation were taken for a point at a distance of one meter. 

 In carrying out the investigation the following precautions were 

 observed, (o) It was found better to work in a room moderately 

 illuminated by a source of light behind the observer and entirely con- 

 cealed from him, rather than in the dark room. The intervals of 

 dark adaptation between exposures in the dark room seemed to make 

 the observer's eye too sensitive for our purpose. This was especially 

 true for certain parts of the periphery of the field of view. In be- 

 coming supersensitive there was a tendency to become erratically 

 sensitive, (b) It was found that blinking serves as a variable factor 

 for the relief of discomfort and that the amount of blinking must be 

 made constant from test to test. This was accomplished by having 

 the observer blink at equal intervals during the exposure, timing 

 himself by the stroke of a metronome. The interval most natural 

 and suitable for this purpose was determined for each observer sepa- 

 rately, (c) All comparisons were planned in series. For example, 

 if it were desired to compare the sensitivity for the temporal and 

 nasal halves of the field of view in a given meridian, the exposure 

 was made first at one point in one half and next at the correspond- 

 ing point in the other half and the order of giving them was changed. 

 This was to guarantee that the eye should be as nearly in the same 

 condition with regard to progressive fatigue, etc., as was possible. 

 Further to safeguard against error in this regard, series were com- 

 pared in which the exposures were repeated in the reverse order. 

 (d) An interval of recovery was allowed between exposures. This 

 interval had to be determined separately for each observer and often 

 had to be made different for the same observer on different days. 

 It was never changed, however, during the course of a series, the 

 results of which were to be compared, (e) In order that the ob- 

 server's head be held rigidly in position during the exposure, he was 

 required to bite an impression of his teeth previously made and 

 hardened in wax on a mouthboard. (/) As has been the case in all 

 of this work, care has been exercised in the choice of observers to 



