Domestic Encyclopædia (1802)/Colour

COLOUR is one of the most remarkable phenomena in nature, the explanation of which, by the ancient philosophers, was vague and unsatisfactory, till Sir Isaac Newton, in 1666, discovered that the coloured image of the sun, formed by a glass prism, was not of a circular, but of an oblong form, contrary to the laws of refraction. Hence he conjectured, that light is not homogeneal, or a simple body, but that it consists of rays, some of which are much more refrangible than others. This theory was very generally received, and subsequently improved upon by Dr. Hooke, as well as by other native and foreign philosophers; and, though the doctrine of colours is far from being determined with sufficient precision, yet we are warranted to admit the truth of the following propositions:

1. All the colours in nature proceed from the rays of light.

2. There are seven primary colours; namely, red, orange, yellow, green, blue, purple, and violet or indigo.

3. Every ray of light may be separated into the seven primary colours.

4. The rays of light in passing through the same medium, have different degrees of refrangibility.

5. The variation in the colours of light arises from its different refrangibility; that which is the least refrangible producing red; and that which is the most refrangible, violet.

6. By compounding any two of the primary colours, as red and yellow, or yellow and blue, the intermediate colour, as orange or green, may be produced.

7. The colours of bodies arise from their dispositions to reflect one sort of rays, and to absorb the other.

8. Such bodies as reflect two or more sorts of rays, appear of various colours.

9. The whiteness of bodies arises from their disposition to reflect all the rays of light promiscuously.

10. The blackness of bodies proceeds from their incapacity to reflect any of the rays of light. Hence it is, that a black body, when exposed to the sun, becomes heated much sooner than any other.

Although, of all sensible qualities, colour is the least useful in ascertaining the virtues and powers of vegetables; yet, as the following general positions have been laid down on this subject, by Linnæus, and as they appear to be sufficiently attested by experience, we shall conclude this article with specifying them.—A yellow colour generally indicates a bitter taste, as in gentian, aloe, celandine, turmeric, and other yellow flowers. Red denotes an acid or sour taste; as in cranberries, barberries, currants, raspberries, mulberries, cherries, the fruit of the rose, sea-buckthorn, and service-tree. Herbs that turn red towards autumn, have also an acid taste; as sorrel, wood-sorrel, and bloody dock. Green indicates a crude, alkaline taste, as in leaves and unripe fruits. A pale colour denotes an insipid flavour, as in endive, asparagus, and lettuce. White, promises to be sweet and luscious to the palate; as in white currants, and plums, sweet-apples, &c. Lastly, black indicates a harsh, nauseous, and disagreeable taste; as in the berries of deadly night-shade, myrtle-leaved sumach, herb-christopher, and others; many of which are not only unpleasant to the taste, but pernicious and fatal in their effects.