1896-97-] SPATIAL THRESHOLDS OF COLOURS. 227 



more difficult to achieve, but quite possible by using colour grounds and 

 employing precautions to secure that the brightness intensity of the 

 ground is the same as that of the colour examined. The devices and 

 detailed means, used to secure this result, cannot be described at all in 

 this preliminary paper, but will legitimately find place in a subsequent 

 paper giving illustrations, detailed description of the apparatus used and 

 our results attained. It may be noticed that a theoretical immunity 

 from both colour and intensity contrast is possible by using a grey 

 ground who.se intensity is the same as the colour examined, but practi- 

 cally it is very difficult, next to impossible to achieve. For, in getting 

 the two intensities equal and constant, you are forced to introduce a 

 more or less influential colour element in the ground ; and in seeking to 

 remove the colour element wholly, it is difficult to avoid introducing an 

 inequality of brightness intensities again. But, be that as it may, there 

 is no question of the scientific desirability of an attempt to limit the 

 influence of contrast phenomena upon the determination of the Spatial 

 Thresholds of colours to specified kinds of contrast, colour contrast at 

 one time and purely light contrast at another, so that the specific effect 

 of each upon the Spatial Thresholds may be discernible from that of 

 the other instead of both being massed in a joint influence in which the 

 contributions of each kind of contrast are indistinguishable with any 

 exactitude. It was, moreover, a matter of scientific importance to make 

 the attempt to rule out tlie contrast influence, both colour and intensity^ 

 altogether, even though actually it may not have been absolutely- 

 successful in doing so. In any case, it will have reduced the contrast 

 influence to a degree so small as to be a minimum up to date in the 

 investigation of the Spatial Threshold of colours. 



In the history of physiological and psychological science this problem 

 of the Space Threshold of Colours has received but scant, almost 

 cursory notice. Where there has been some approach to a scientific 

 determination of it quantitativ^ely, it has been marred by a twofold 

 defect. On the one hand, the investigation has been carried on by 

 means of appliances which are extremely crude and inaccurate, and on 

 the other hand the inquiry has been wholly confined to exploiting only 

 one kind of contrast influence, namely, light intensity, which may ha\e a 

 bearing on the Threshold. It has wholly ignored the roles which other 

 kinds of contrasting environs, such as the various colours of our ordinar\- 

 colour system maj^ have, and practically do have, upon our perceptibility 

 of colours, according to their size of exposure as relative to the 

 spectator. It has, moreoxer, apparently, not seen the desirabilit}' of 

 modifying extremes in light intensity contrast and of obtaining.if possible, 



