In consequence of this contrast, the stripes seen from a proper distance, resemble channels rather than flat surfaces. For, in the stripes 2 and 3 for instance, the grey is weakened from the edge _a a_ to the edge _b b_, presenting to the eye the same effect as if the light fell upon a channelled surface; there is however this difference, that in the real channelling the enlightened part would throw a reflection upon the dark portion. 12. Contrast of tone occurs with colours so called as well as with grey; thus to repeat the experiment (9), fig. 1, with the halves _o o_ of a sheet of paper of a light tint of a certain colour, and the two halves _p p_ of a sheet of paper of a darker tint of the same colour, it will be seen that _o_ contiguous to _p_ will be lighter than _oʹ_, and _p_ darker than _pʹ_. In short, it may be demonstrated as has been done (10) that the modification of colours in juxtaposition becomes weaker in proportion to their distance from the line of contact; and in order to observe this effect in bodies which are not contiguous, it is only necessary to experiment as described in (10). [Illustration: PLATE II.] The colours experimented upon must be as nearly as possible of equal intensity. 13. _Experimental Demonstration of Contrast of Colour._—If we arrange as before, the two halves of an unglazed coloured sheet of paper, and two halves of another sheet of a different colour, but as nearly as possible of equal intensity, or rather of _tone_ (8), upon looking at the four half-sheets _o oʹ, p pʹ_ for a few seconds, we shall see that _o_ differs from _oʹ_ and _p_ from _pʹ_; consequently the two half-sheets, _o p_, seem to undergo a reciprocal modification of tint, which is rendered apparent by comparing their colours with those of _oʹ_ and _pʹ_.