CHAPTER I.

INTRODUCTION.

(501.) HAVING made it a rule in this work never to state any observations which I have not myself verified, whenever I do not quote the name of the author, I must mention that, not possessing every requisite for the examination of the subject of this section, I am obliged to develop certain points only at the same time I shall indicate those which I have not treated as I desired.

(502.) It is not possible to pass a sound judgment on the different assortments of colours with respect to the use which can be made of them in reading, whether of printed or written characters, or by any other means, so far as regards

A. The duration of the reading;

B. And the kind of light which illumines the printed or written paper.

A. INFLUENCE OF DURATION IN THE READING.

(503.) From the different conditions in which the eye is found when it is apt to perceive the phenomena of simultaneous, successive, and mixed contrasts of colours (77. and following), we conclude, that, in order to judge of the effect upon the sight of the assortments we can make between the colour of the letters and that of the paper, with regard to the degree of facility that different assortments respectively present in the reading, it is necessary to take into consideration the length of time during which we read; for it may happen that one assortment will be more favourable than another during a brief reading, while the contrary will take place if the reading be prolonged during several hours; besides, the first assortment, presenting the greatest contrast to the second, will, by the same reason, be more favourable in a reading of short duration, while it will be less so in a prolonged reading, because then, in consequence of the intensity of its contrast, it will fatigue the eye more than the second.

B. INFLUENCE OF THE KIND OF LIGHT WHICH ILLUMINES PRINTED OR WRITTEN PAPER.

(504.) 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 when they are illumined by this latter light, it is evident that if we neglected this difference of relation it would give rise to error, because any assortment of colours which might be more favourable to the reading under diffused daylight might be less so in the light of a candle, lamp, &c.

(505.) Conformably to the distinction I have just established, I am about to examine in the two following Chapters:

1o. The influence of different assortments of colours which we may make use of in writing and printing to render more or less easy a reading of characters printed or traced in any manner, continued for some minutes or hours, by diffused daylight.

2o. The influence of the same assortments when they refer to a short or prolonged reading made by artificial light.

CHAPTER II.

ON THE ASSORTMENT OF COLOURS WITH RESPECT TO READING BY DIFFUSED DAYLIGHT.

ARTICLE 1.

Reading of a few Minutes' Duration.

(506.) THE reading of letters printed or written upon paper is done without fatigue only where there is a marked contrast between the letters and the ground upon which they are presented to the eye. The contrast may be of tone or of colour, or both.

Contrast of Tone.

(507.) 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 gradation from normal Grey; in fact, Black letters upon a White ground present the maximum of contrast of tone, and the reading is made in a perfectly distinct manner, without fatigue, by diffused daylight, affording the proof of what I advance: finally, all those whose sight is enfeebled by age, know how the want of light, or, what amounts to the same thing, how the Grey tint of paper, in diminishing the contrast of tone, renders it difficult to read the letters which they could well do without difficulty in a vivid light, or, what is the same, upon papers Whiter or less Grey than that of which we speak.

(508.) Black characters upon Grey paper are difficult to read: this is the reason we do not print or trace with a coloured ink upon paper of the colour of this ink, even if there be a great difference between the two tones.

Contrast of Colour.

(509.) To appreciate the influence of this contrast, the colour of the letters and that of the paper must be taken at the same height of tone, in order to perceive the effect of mutual contrast of two colours only.

(510.) According to the distinction we have made of luminous and sombre colours in equality of tone (184.), it is evident that the contrast most favourable to distinct vision will be that of a luminous colour, such as Red, Orange, Yellow, with a sombre colour, such as Violet, Blue, and that in this case the effect will be at its maximum, if the colours are complementary, as Orange and Blue, Yellow and Violet, &c.

(511.) I have already remarked that Red and Green afford a complementary assortment which presents the least contrast of luminousness, because under this relation the Red is placed between the elements of Green, of which the one, Yellow, is the brightest colour, and the other, Blue, is the most sombre (187.). Thus, Red and Green are comple

mentary colours, the least suitable to be opposed to each other in writing or printing coloured characters upon coloured grounds.

Contrast of Tone and of Colour.

(512.) If the contrast of Black and White is the most favourable to distinct vision, and if the contrast of two colours taken at equal height of tone is favourable only in the degree of one being sombre and the other luminous, then we must necessarily conclude that, whenever we would wish to deviate from the opposition of Black and White, we must at the same time make a contrast of tone and of colour, otherwise the reading of letters which are not in this condi tion of contrast with their ground, will be fatiguing or difficult.

(513.) Next to the opposition of Black with White, come those of Black with the light tones of luminous colours, such as Red, Orange, and Yellow; then those of these same light tones with deep Blue.

(514.) The opposition of luminous colours such as those of Red and Orange, of Red and Yellow, of Orange and Yellow, yield nothing favourable to view: it will be better, I believe, to avoid their associations.

(515.) In all the preceding, I have spoken only of the opposition of tone and colour existing between the letters and the ground upon which we read them; it remains for me to treat the questions whether it is advantageous for the reader that the letters be darker than the ground, as they generally are in printing and writing with Black ink upon white and coloured paper, or whether the reverse is preferable; or, finally, if the two cases present equal advantages. Not having had at my disposal every requisite for resolving these questions, I have not occupied myself with them. I shall limit myself to the single remark that, the letters presenting much less extent of surface than the paper which serves as their ground, there is a superiority of clearness in the particular assortment generally adopted, and clearness is always favourable to distinct vision.

(516.) I will now give some examples of Black characters

printed upon coloured paper; commencing with those which appeared to me the easiest to read.

1o. Black characters upon White paper.

2o. Black characters upon light Yellow paper. 3°. Black characters upon light Yellow-Green paper. 4°. Black characters upon light Orange paper. 5°. Black characters upon light Blue paper. 6°. Black characters upon Crimson-Red paper. 7°. Black characters upon deep Orange paper. 8°. Black characters upon deep Red paper. 9°. Black characters upon deep Violet paper. I shall remark that I read almost as well upon light Orange as upon light Yellow-Green paper.

(517.) I have every reason to believe that other eyes than mine would require some change in the order I have assigned to the preceding assortments.

ARTICLE 2.

Reading of some Hours' Duration.

(518.) The order which may be established between different assortments of colours relatively to the greater or less facility which they respectively present for a reading of some minutes' duration will doubtless differ among some persons from the order in which the same persons would range them in a reading of some hours' duration.

Thus, there may be such an assortment of black letters with coloured paper which, being less favourable to a reading of a quarter of an hour than the assortment of black letters on white paper, will be preferred to this latter by a person to whom the contrast of black and white, seen during many hours, would cause more fatigue than would be occasioned by reading the same letters upon yellow, green, blue, &c., paper, properly selected both as to the height of tone and hue. Unfortunately, I have not been able to make comparative proofs sufficiently prolonged for me to state positive results; for I have only had at my disposal a few loose sheets of paper of different colours upon which black characters were printed.