230 TRANSACTIONS OF THE CANADIAN INSTITUTE. [VOL. V. 



are described in his article, " Ein Photometrescher Apparat Zu 

 Psychophys. Zwecken," some of whose results described therein I 

 subjoin : 



I. Paris Black : 



(t) in lampligfht (petroleum) is -V-s ^'^ h of white. 



(2) in gfaslig-ht , 5V of white. 



(3) in diffuse daylight ^V.^ of white. 



II. China Ink : 



( 1 ) in lamplig-ht 77*5. u of white. 



(2) in diffuse daylig:ht ^V^ of white. 



III. Graphite (F"aber B.B. Pencil): 



( 1 ) in lamplig-ht ^.^ of white. 



(2) in diffuse daylight i.^ of white. 



IV'. A specimen of black cardboard in sunlight was jV of the intensity of white 

 paper in sunlight. 



It consequently follows that, unless we employ some means of 

 specifying the particular black which is used in the ground, the results 

 obtained can have only a private importance and not presume to be 

 representative of the space threshold judgments of colours on a black 

 ground. For it is evident that with two variables in regard to 

 brightness, such as the colour pigment, and the unspecified black ground 

 used must be, it would simply be impossible to know to what degree the 

 influence of intensity contrast participated in the determination of 

 the thresholds for any one colour, much more to what different degrees 

 it entered in the determination of the thresholds of the other colours. 

 It might apfiear that it is sufficient if only the colour and ground are 

 subjected to the same illumination, for, at first sight, it might seem that 

 this would secure a constant brightness relation between ground and 

 colour. But a moment's reflection shows us that this would fail to 

 ensure the constancy of this contrast relation throughout the various 

 colours because of the fact that different pigments under the same 

 conditions of illumination reflect light in varying degrees. Hence, if the 

 illumination remained constant there would be a variation in the light 

 intensity relation of colour to ground, for each of the colour pigments 

 used, which would, of course, preclude the possibility of there being a 

 quantitatively uniform contrast influence throughout, contributing to the 

 space threshold determination of the various colours. This taken in 

 connection with the fact that there is no clue to knowing the 



