426. The light we employ to supply the place of that of the sun, changing the relations of colour under which the same bodies appear to us illumined by daylight, it is evident that if we neglected this difference of relation it would give rise to error; because any assortment of colours favourable to read in diffused daylight, might be less so by the light of a lamp, &c. 427. I will now examine— The influence of different assortments of the colours of writing and printing for reading by diffused daylight. ON THE ASSORTMENT OF COLOURS FOR READING BY DIFFUSED DAYLIGHT. _Reading of a Few Minutes’ Duration._ 428. Letters upon paper can be read without fatigue only when there is a marked contrast between the letters and the ground. This contrast may be of tone, or of colours, or both. 429. _Contrast of tone_ is the most favourable condition for distinct vision, if we consider white and black as the two extremes of a scale, comprehending the gradations from normal grey; in fact, black letters upon a white ground present the maximum of contrast of tone, and may be read in a perfectly distinct manner without fatigue by diffused daylight. Indeed, all whose sight is enfeebled by age require the utmost contrast of tone. FOURTH DIVISION. _Employment of Colours in Architecture._ I. ON THE EMPLOYMENT OF COLOURS IN EGYPTIAN ARCHITECTURE. 430. The Egyptians employed various colours, as red, yellow, blue, green, and white, to decorate their monuments. Lancret remarks, “All who have seen Egyptian paintings, can attest that when seen, even for the first time, they were not disagreeable; and, that if at first the colours appear distributed arbitrarily, it is because observers have not combined a sufficient number of observations, and that it will one day be found that this part of the arts of the Egyptians was, like all the rest, submitted to invariable rules.” 431. Champollion the Younger expresses himself in these terms on the application of colours to Egyptian architecture: “I should like to introduce into the great temple of Ipsamboul, all who refuse to believe in the elegant richness that painted sculpture adds to architecture; in less than a quarter of an hour, I engage that they would perspire away all their prejudices, and that their _à priori_ opinions would quit them through every pore.”