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



an environing ground which shall practically exercise a very small or 

 even no appreciable contrast influence of any kind upon the examined 

 colour. But it is evident that without so doing no approximation could 

 be made to a discovery of the precise degree to which the influence of 

 contrast modified the Spatial Threshold. 



Quite early the phenomenon of the dependency of colour perception 

 upon intensity and on the visual angle of the coloured surface was noted 

 by Purknije in his Commentatio de examine organi visus 1823. 

 Plateau shortly after in Poggendorff's Annalen, 1830, noted the 

 disappearance of colour tone altogether at small visual angle. However, 

 the problem was not dwelt upon, perhaps not appreciated by these early 

 investigators. No effort was made to make an exact inquiry into 

 the relation of visual angle and colour perceptibility until the time of 

 Von Wittich and Aubert, both of whom made some important pioneer 

 attempts in this direction. Von Wittich distinguished the two Spatial 

 Thresholds which we have mentioned, namely, the light, or achromatic 

 threshold and the chromatic threshold, and he sought to obtain a 

 quantitative representation of the same. He conducted his experiments 

 upon a black and white ground respectively, thus really taking into 

 account the fact of a contrast influence in the determination of the 

 thresholds. But apparently, as we have indicated, he did not 

 appreciate the desirability of extending the enquiry to include other 

 kinds of contrast. Much less did he think of determining what role the 

 contrast, as such, played in the matter, by seeking to reduce it to 

 a minimum or to remove it so far as intensity and colour contrast, 

 altogether. 



His experimental work on this question was crude in many other 

 ways than in the scope of its attempt Its physical appliances, such as 

 the means used to obtain colours and grounds, its measurement methods, 

 were all of the crudest and are consequently unsatisfactory in the 

 results yielded thereby. The colours which he experimented on were 

 simply ordinary pigment papers which in these latter days, at least, are 

 well-known not to be pure specimens of the colours which they are 

 taken to represent. For the most part, on the contrary, they contain 

 very many other colour elements obscurely fused vvith the one which, 

 from its predominance, we call the colour of the pigment. In some 

 cases the pigment paper may owe its special colour to the fact that 

 it contains all the other elements of the colour system except the 

 complementary of the colour after which we name it. For example, the 

 most yellow pigments owe their colour to the fact that they contain the 

 whole spectrum of colours except the blue, and even in somecases this 



