60 COLOUR VISION 



colour. Ebbinghaus^ and Christine Ladd-Franklin^ almost simul- 

 taneously drew attention to the significance of the facts. They showed 

 that three whites made by mixture of red and blue-green, yellow and 

 blue, and green-yellow and violet darken unequally with proportionally 

 diminishing intensity, the first least, the second more, and the third 

 most. Konig's experiments^ with complementary colours are confirma- 

 tory. Still more so are observations on the colour-blind. 



On the other hand as soon as dark adaptation is sufficiently great 

 to abolish the sensations of colour any alteration of the intensity of 

 the light which does not exceed the colour threshold fails to alter the 

 various matches. It is true that Stegmann^ found slight differences, 

 but they were too slight to be of much importance, and such as 

 they were were in a direction opposite to that taken in the Purkinje 

 phenomenon. 



As Lummer^ pointed out, the peculiarities of scotopic vision explain 

 an observation of Williams^, H. F. Weber'^, and Aubert*^ that a body 

 heated to redness in the dark first shows a grey glow. This occurs 

 at 400° C. (H. F. Weber ; 379° C, Gray^) ; as the temperature rises the 

 yellow-green rays increase and cause a yellowish-grey glow. At about 

 525° C. (Draper) the red glow commences, but the temperature varies 

 with the conditions of the experiment. If the observer is light-adapted 

 these preliminary stages are invisible. Abney^° had previously recog- 

 nised the explanation of these facts. 



From what has already been said we see that there are two thresholds 

 of vision — a general threshold, the minimal stimulus producing the 

 sensation of light ; and a specific or colour threshold, the minimal stimulus 

 producing the sensation of colour. The interval between them is 

 known as the colourless or photochromatic interval. It depends upon the 

 scotopic visibility of .the given light below the threshold of photopic 

 vision, and varies with the condition of adaptation and the nature 

 of the light stimulus^^ The colourless interval increases with in- 

 creasing dark adaptation, and this is due to lowering of the general 



^ Ztsch. f. Psycliol. u. Physiol, d. Sinnesorg. v. 145, 1893. 



- Nature, XLvm. 517, 1893. » Kcinig, p. 373. 



* Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxv. 226, 1901. 



* Wiedemann's Ann. LXii. 14, 1897. '^ Pogg. Ann. xxxvi. 494, 1835. 



"> Wied. Ann. xxxii. 25G, 1887. ** Physiologic der Netzhnuf, p. 41, 1865. 



9 Proc. Phys. Sac. xiii. 122, 1894. 

 ^° Colour Vision, Tyndall Lectures, p. 35, 1895 



1^ Nagel and Schiifer, Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxxiv. 271, 1904; 

 Loeser, Ibid, xxxvi. 1, 1904. 



