2. Of the very different manner in which not only different people, but even the same person, will judge of the colours of a more or less complex pattern, having a certain number of colours, according to the attention the spectator gives at a certain moment to different parts (407). 423. Our examination of the border of roses with their leaves (No. 3), and especially of that of the border of flowers varied in their forms and hues (No. 4), shows the necessity of a knowledge of the law of contrast to assort the colours of objects represented upon a border with the colour which serves as a ground to them. The examination of the border No. 4 has demonstrated experimentally that this assortment presents so much the more difficulty as we wish to have purer tints for the ground, and more varied colours in the objects we intend placing on it; besides, in demonstrating the good effect of grey as a ground for these latter objects, it has furnished an example of a fact which may be deduced from the law, and which is in perfect accordance with what practice taught us long ago. PRINTED OR WRITTEN CHARACTERS ON PAPERS OF DIFFERENT COLOURS. 424. Having made it a rule in this work never to state any observations which I have not myself verified, I must mention that, not possessing every requisite for the examination of the subject of this section, I am obliged to develop certain points of it only. We must regard— 1. The duration of the reading, and 2. The kind of light which illuminates the printed or written paper. A.—INFLUENCE OF DURATION IN THE READING. 425. From the different conditions in which the eyei s found when it is apt to perceive the phenomena of simultaneous, successive, and mixed contrasts of colours (77 _et seq._), it may be conceived that in order to judge of the effect upon the sight of the assortments of the colour of the letters and that of the paper as to the degree of facility that they respectively present for reading, it may happen that one assortment will be more favourable during a brief reading, while the contrary will take place if the reading be prolonged during several hours. Besides, an assortment presenting the greatest contrast will be more favourable to a reading of short duration, while it will be less so to a prolonged reading; because, in consequence of the intensity of its contrast, it will fatigue the eye more.[2] [2] Hence the pleasure of reading books printed on the modern _toned_ paper. De Morgan advises that tables of logarithms, and the like, should be printed on pale brown paper. B.—INFLUENCE OF THE KIND OF LIGHT ON PRINTED OR WRITTEN PAPER.