Photometry of Lights of Different Colours, 747 



brightness curves are grouped about it in a manner to suggest 

 that it is the true normal of which they are all distortions. 



The second set of experiments performed was to study the 

 effect of changing the surroundings of the photometric field. 

 It was suggested to the writer by a psychologist that the 

 ordinary optical instrument using a telescope subjects the eye 

 to an abnormal condition in that the field of the instrument 

 is surrounded by a totally black space. Under normal con- 

 ditions, on the other hand, the whole retina is active. Such 

 an abnormal condition might be expected to disturb such an 

 unstable thing as an equality of brightness judgment. 



In order to study this problem the telescope tube of the 

 spectrometer was removed. The eye-slit was attached to a 

 separate rigid stand, the object-glass was held in its original 

 position by a different means. Against the objective were 

 laid large flat metal plates pierced by openings of a size to 

 give the same effective photometric fields as in the previous 

 experiments. These plates were smoked with magnesium 

 oxide and arrangements were made to illuminate them, when 

 in place, by two incandescent lamps whose intensity could be 

 controlled by a series resistance. The plates were of such 

 size that upon turning on the incandescent lamps the whole 

 field of view, except as limited by the ocular slit, was bright. 

 This meant a field of approximately fifty degrees and was 

 considered ample to test the matter in question. 



A series of luminosity curves under the same conditions as 

 previously obtained were made, except that both light and 

 dark surroundings were tried. Again the results were in 

 marked contrast as obtained by the equality of brightness 

 and flicker methods. By the former method different curves 

 result depending on the character of the surrounding field. 

 But here, as before, no systematic relationship exists in the 

 curves under the two conditions. The effect of changing 

 the surrounding field from dark to light is to introduce a 

 shift or distortion which may apparently be in either direc- 

 tion. It is again a case of disturbed judgment. In fig. 1 

 the effect of a light surrounding field as found in one instance 

 is given. With the flicker method, on the other hand, 

 changing the surrounding field introduces no variations 

 certainly larger than the errors of setting. 



Before leaving the experiments on light and dark sur- 

 roundings some incidental results may be noted. With the 

 small field, using the equality of brightness method, a very 

 considerable increase in definiteness of setting was found 

 with the bright surrounding field. A similar increase was 

 not found with the large field, nor by the flicker method, but 



