1896-97-] SPATIAL THRESHOLDS OF COLOURS. 23! 



specific character of the black ground used by Von Wittich, would 

 render his results of small value for scientific purposes, because there is 

 even no ground for forming comparison as to the relative magnitude of 

 the space threshold for the various colours. 



In our own experiments, I may say, we sought to remove these 

 deficiencies in the work of Von Wittich by securing, on the one hand, an 

 absolute black, or as near an approach as practicable, and on the other 

 hand, by ensuring that the brightness intensities of the colours were so 

 regulated that their relative brightness as compared with the ground 

 was constant throughout their entire range. How this end was 

 practically achieved we do Jiot propose to describe at length in this 

 present paper, but, suffice it to say, that we were successful in attaining 

 an approximation to this constant intensity contrast relation which was 

 substantially complete. This was done, on the one hand, by using 

 a black ground which was black because of the exclusion of light from 

 it, and was viewed through a long darkened tube with a diaphragm at 

 the end, the inner surface of which diaphragm constituted the black 

 ground, and the adjustable aperture of which became the coloured 

 surface by reason of the coloured light entering through it. On the 

 other hand, we refused to employ daylight illumination because of its 

 uncontrollable and inconstant intensity, but used instead incandescent 

 electric lamp illumination which by controlling the distances of the lamp 

 from the different coloured surfaces illumined in such a way that their 

 brightness was in each case judged equal to that of another standard 

 illuminated surface, gave us a series of position marks of the lamp for 

 the respective colours wherein their intensities as light were all equal. 



After Von Wittich, Aubert took up this same problem, and, with a 

 few improvements, followed largely in the footsteps of his predecessor, 

 repeating to a large extent the same errors which we have noticed. 

 Like Von Wittich he used ordinary pigment papers for his colours 

 without any correction of the inaccuracies incident to their use, and for 

 his black ground ordinary black cardboard. As we should expect, his 

 results do not agree with those of Von Wittich at all, which only 

 goes to show the truth of what we have already pointed out in refer- 

 ence to Von Wittich's results, viz.: that they are true only of the 

 particular pigments which Von Wittich used, of private and not general 

 validity. 



The method of observation and measurement of the thresholds, 

 employed equally by Von Wittich and Aubert, was also very crude. 

 They took coloured objects and removed themselves gradually away 



