Those diversified colours which render the floral world so attractive, which impart such beauty to the feathered tribes and the legions of butterflies, and in brilliant pigments re ward the labours of the chemist, are not pro perties inherent in matter itself, but arise from the action of matter upon light, whereby certain of the coloured rays which form white light are reflected, whilst others are absorbed or transmitted. Scarlet cloth, for instance absorbs almost all colours except red, which it reflects ; but those substances which reflect all the rays appear white ; those which ab sorb all are black. The brilliancy of tints is greatly increased when viewed in light of their own colour, as may be proved by throw ing the red rays from a prism upon any scarlet object, or the green rays upon a green leaf. The colour of transparent substances depends upon their property of absorbing some of the colours of white light, and transmitting others. The blue tint of the atmosphere in reflected light, and its red morning and evening tinge, are to be ascribed to this cause.
Dyes and paints are substances which, when applied to bodies, so change their sur faces, that when seen in white light, they re flect only the particular colour of the dye or paint. There are several modes by which white light can be reproduced, of which a simple one is, the rapid rotation of a disk painted in stripes, with the prismatic colours in the correct proportions. In this case, the eye receives the impression at the same time, and in the same place, of a red circle, an orange, a yellow circle, and so on, and conse quently a white circle, since the sensation of white is but the simultaneous sensation of all these colours.
As in the production of white light, it is necessary that all the simple colours should exist in their due proportions; so it is evident that by suppressing or increasing one the harmony will be destroyed, and the light will be no longer white. Thus, for instance, by suppressing red, we obtain a blueish green, which, compounded with red, would form white light. Whenever two colours, simple or compound, fulfil this condition, they are said to be complementary one to the other. They are as follows :— their absolute transparency, but also on the density of the medium through which the light passes. There is no perfectly opaque or perfectly transparent substance known. Diamond is nothing more than charcoal in a different state of molecular aggregation, and gold can be made pervious to light. On the other hand, the purest air or clearest water gradually extinguishes by absorption the rays transmitted through them. According to Bouguer the purest sea water loses all its transparency at a depth of 730 feet, and the reason that more stars are visible from the summit of a lofty mountain than from the level of the sea is, because the light from the more distant stars becomes so much enfeebled during its passage through the lower strata of the atmosphere, that it has not sufficient power to affect the sight.
If a pencil of rays diverging from a lumi nous point fall upon the surface of a convex lens, they will not all be equally refracted. The ray which passes through the axis of the lens will not be changed in its course, but the remainder of the rays will be more and inore refracted in proportion as they recede from the optical centre of the lens. When the rays pass out from a bi-corivex lens into air they are refracted from a line perpendicular to the point of emergence: the effect is to cause them all to converge towards the cen tral ray to a point at which they meet, called the focus. The distance between the focus and the refracting surface is the focal dis tance or focal length, and is influenced by the refracting power of the lens, the amount of its curvature, and the distance of the lumi nous body.
Parallel rays entering any plano-convex or double convex lens at an equal distance from its axis, are concentrated to the same focal point, but as the peripheral rays are more refracted than the central rays, they are sooner brought to a focus; hence the image formed at the focus of the lens is somewhat indistinct at its edges. This imperfection is• due to what is termed spherical aberration, and is counteracted either by shutting out the peri pheral rays, or by such a combination of lenses as will establish a just proportion between the refraction of the central and peripheral rays. Such lenses are made of crown glass, com posed of flint and alkali only, and flint glass, in which oxide of lead is added to the other materials. The latter possesses a much higher dispersive power than the crown glass; but the refraction of the rays is nearly the same in both, and when combined, achromatic lenses are obtained. This term is applied from their utility in obviating another source of con fusion, chromatic aberration, which is caused by the unequal refrangibility of the prismatic rays when transmitted through an ordinary lens, whereby the images are fringed with colours, and are rendered even more indis tinct than by spherical aberration. Newton supposed that an achromatic lens was an im possibility ; but in 1757 Dollond completely succeeded in overcoming the difficulty by the 'With respect to the production of light, bodies are divided into luminous and non luminous ; among natural bodies some pos sess in themselves the property of exciting in our eyes the sensation of brightness, or light, as the sun and other heavenly bodies which shine by their own light. There is also chemical light, or that produced by com bustion, electric light, phosphorescent light, et cetera. Non-luminous bodies are such as become visible only when light fills upon them from some luminous source.