RAINBOW.
This appearance is also seen by moon light, though seldom vivid enough to render the colours distinguishable; and the artificial rainbow may be produced even by candle-light, on the water which is ejected by a small Ibuntain, or jet d'eau. All these are of the same nature, and de pendent on the same causes, viz, the va rious refrangibility of the rays of light.
The colours observable on soap•bub. tiles, and the halos which sometimes sur round the moon, are also referable to the same origin.
"Of the Eye, and the nature of Vision." The eye is nearly of a spherical shape, and is composed of three different sub stances, called, 1. The aqueous, P. (fig. 17). 2. The crystaline, R; and 3. The vitreous humours, V, enclosed by three principal coats, which are formed by the expansion of the different component parts of the optic nerve, viz. the sclerotica, S. S. 2. The choroides, I) D ; and 3. The retina, T 1'. The sclerotica is outermost ; it is very strong, and the forepart, which is transparent, and somewhat prominent, is called the cornea, C. The choroides is next in order, and has a circular perfora tion, P, called the pupil, immediately be hind the middle of the cornea : the part II. of the choroides, visible behind the cornea, is flat ; it is called the iris, or uvea, and is differently coloured in different persons. The retina is the inmost coat, it extends round the eye till it meets the ciliary ligaments, QQ, membranes pro ceeding from the choroides, and attached to the capsula or filament, which encloses the crystaline humours, R. The crysta line is the most dense of the three hu mours, and is in the shape of a double convex lens, whose forepart has the less curvature ; the cavity between the cornea and the crystallite is occupied by the aqueous humour, which has rather the least density of the three, and the space between the bottom of the eye and the crystaline is filled by the vitreous humour, V.
Objects presented to the eye have their images painted on the back part of the retina, the rays of the incident pencils converging to their proper foci there by the refraction of the different humours ; and for this office they are admirably adapted; for as the distance between the back and front of the eye is very small, and the rays of each of the pencils that form the image fall parallel, or else diverging on the eye, a strong refractive power is necessary for bringing them to their foci at the retina ; but each of the humours, by its peculiar form and densi ty, contributes to cause a convergence of the rays ; the aqueous from its convex form ; the crystaline by its double con vexity and greater density than the aqueous; and the vitreous by a less density than the crystaline joined to its concave form.
These things are manifest from what has been already said. The structure of the eye is in general adapted to the re ception of parallel rays ; but as the dis tances of visible objects are various, so the eye has powers of accommodating itself to rays proceeding from different distances, by altering the distance of the crystaline from the retina, which is done by the action of the ciliary ligaments.
That this change of situation in the crystaline is adequate to such accommo dation, may be thus shewn. Suppose a pencil of rays to diverge from a point, A, (fig. 18.) at a distance from the eye less than that which admits distinct vision in the usual situation of the humours : the rays would come to a focus, V, behind the retina, L M. Let the crystaline, 0 P, be brought forward, and, C V, the dis tance of the focus from the crystaline, will be increased ; but, because of the great proportion that A C, the smallest distance that admits distinct vision has to F C, the focal length of the crystaline, the distance, C 0 of the crystallite from the retina will be more increased than 0 V, so that C G and C V may be come equal, and thus the focus made to fall exactly on the retina.