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



rom them until the colour quality of the objects began to change, and 

 finally disappear from the spectator. Then they gradually approached 

 again noting carefully their distance from the coloured object when it first 

 became visible, and second, when it first became visible as coloured. 

 Now it must appear that such a method of procedure, besides being 

 subject to great inaccuracies in measuring the slight transitions of dis- 

 tance, often sufficient to cause a great change in the appearance of the 

 coloured objects, is open to the serious objection of not providing for 

 the influence of anticipation in lowering the threshold judgments of 

 the colour as seen in their proper tone. According to this method the 

 observer is familiar with the colour to expect, and this fact must of 

 course materially modify the ease with which he detects the colour tone 

 of the object gradually growing into view. In our work we avoided 

 both these imperfections, viz. : of crude measurement methods and 

 threshold modification from anticipation. We performed our experi- 

 ments in such a way that the observer was quite in ignorance as to 

 what colour in each case he might expect. The result was, of course, 

 that the observer had to decide among the various spectral colours and 

 their transitions quite upon their own merits and not according to 

 information previously possessed as to what might be expected to 

 appear. On the other hand, instead of employing the gradual departure 

 or approach method of varying the visual angle of the coloured objects, 

 we adopted the plan of keeping the distance between object and 

 observer constant and varying the size of the coloured object through 

 gradual transitions. In this way, by a very skillful contrivance 

 described elsewhere, we were enabled to measure the transitions in size 

 with minute accuracy to at least ^u^oirth of an inch, or even closer if 

 desired. 



As was to have been expected, our experimental results on the black 

 ground, do not tally with those of Von Wittich or Aubert, who likewise 

 do not correspond in results with each other. It would have been 

 rather peculiar, in fact, if it had come about that our results corresponded 

 with theirs, especially where peculiar care was taken to remove the 

 several inaccuracies which we have noted in their mode of procedure. 

 With the removal of the adulteration of tones in the colours experi- 

 mented upon, and the regulation of a variable uncertain contrast influ- 

 ence, such as played an important, though unsuspected, role in their 

 work, to the end that it became a constant quantity with us, and with 

 the specially superior accuracy of our measurement apparatus, it would 

 have been remarkable indeed had the results not differed materially 

 from both Aubert's and Von Wittich's who took none of those 



