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the day-time produces two shadows, and one of them of a most beautiful blue colour.
The experiment is the more valuable, as it may be made at any time of the day with a
burning candle. Almost darken a room, and then by means of a lighted candle and a
little day-light, produce two shadows of any small object, as of a pencil, &c. one from
the candle, and another from the daylight received at a small opening of one of the
window-shutters; the light of the candle will appear orange-coloured in the day-time,
and so will that shadow of the body which belongs to, or is made by the day-light;
but the shadow of the body made by the candle, will surprise any person, by being of
a fine blue.
19. More than once I have been agreeably struck with this appearance produced
unintentionally when I have been writing by candle-light-in a winter's morning; upon
the day-light being let in, the shadow of my pen and fingers in the orange-light of the
candle, were beautifully blue.
2(). I suppose there is such a thing as the harmony of colours, of which painters
speak so much; according to the explanation here given, our key to the solution of
every case of harmony and of contrast, is to consider what is the other-colour, simple
or compound, which, joined to a given one-simple or compound, will constitute
white. Thus red, requires green; yellow, purple; blue, orange; and vice versA, the
mixtures in proper proportions will be white.
,1. Sir Isaac Newton (Prop. 6. part, 2. of book i. Optics.) has given a method for
judging of the colour of the compound in any known mixture of primary colours, but it
is not easy, even for mathematicians, to put his rules in practice. The gentleman who
consulted me on this subject of shadows, has been accustomed, for a long time, to assist
his memory when he is painting by the use of the following simple diagram. Let R.Y. B.
represent the three uncompounded colours, red, yellow, blue; and let O. G. P. repre-
sent the compounds, orange, green, purple. It is evident, that to make a deeper
orange, we must add more red; and to make a bluer green, we must add more blue;
and to make the purple redder, we must add more red, and vice versA: but besides
this, the diagram puts us in mind that G. is the contrast to R. and that therefore those
two colours cannot be mixed without approaching to a dull whiteness or greyness;
and the same may be said of Y. and P. and of B. and O.; these colours are also con-
trasts to each other, by mixture they destroy each other, and produce a whiteness4
or greyness, according as they are more or less perfect, but when kept distinct, they
are found to make each other look more brilliant by being brought close together,
and all this is agreeable to what is said in Sect. 1 1. and in the Note to Sect. 14.
2Q. Sir Isaac Newton observes, that he had never been able to produce a perfect
white by the mixture of only two primary colours, and seems to doubt whether such
.a white can be compounded even of three. He tells us, that one part of red lead,