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



inverse ratio. There is a little difference caused by the aerial 

 perspective according to which coloured surfaces of greater distance 

 have to appear less saturated and even naodified in colour on account of 

 the imperfect translucency and bluish colour of the air. This applies to 

 the ship. Consequently the Spatial Threshold of colours will always 

 be measured by the visual angle of the coloured object, where we 

 say it is at the threshold of discernment as such or such a colour. 



We have mentioned the fact that a coloured surface at very small 

 angles, gradually, in most cases, loses its characteristic colour and 

 becomes coloured in another tone, and at smaller angles still, loses its 

 chromatic quality altogether and appears a.s merely colourless light. It 

 thus appears that there will be three distinguishable Spatial Thresholds 

 for any colour. (i) The main one which we may designate the 

 Characteristic Colour Threshold, where the colour is first seen correctly 

 in its proper tone, (2) the Chromatic, where it is first visible in any 

 colour whatever different from colourless light, and (3) the light or 

 Achromatic Threshold, where a coloured object is first perceivable at all, 

 not necessarily as coloured but as colourless light. It appears desirable 

 to keep these three Thresholds separate, though, of course, they can 

 quite easily be investigated concurrently, because, in determining the 

 main Threshold as to the visibility of colours in their proper tone it is 

 necessary, starting from zero, to pass through successively the two minor 

 Thresholds. Hence, the one set of experiments can with care be made 

 to exploit at the same time the three Spatial Thresholds of colours. 



A moment's reflection will show us, moreover, that the problem of the 

 Spatial Threshold of colours cannot be prosecuted except under the 

 conditions of the phenomena of contrast, to a greater or a less degree. 

 We never, as a matter of fact, can get our colours except as they are 

 environed by surfaces which exercise a more or less effectual contrast 

 influence upon them, whether it be merely a light contrast or a satura- 

 tion contrast {e.g., grey of equal intensity with the ground) or ordinary 

 colour contrast, or a combination of two of them or of all three. But 

 although apparently impossible to escape altogether from the conditions 

 of contrast, it was, undoubtedly, a scientific desideratum to free our 

 experiments from an irregular and uncontrolled participation of contrast 

 influence. It is quite possible to eliminate large elements of a mixed 

 contrast influence and to confine its operation to that of either simply 

 intensity contrast, where colour contrast has been removed, or of colour 

 contrast simply where intensity or light contrast has been removed. 

 The former is possible by employing only colourless grounds on which 

 our colours are seen, grey, white or black grounds ; the latter is much 



