of the bubble, till they vanish in the same order in which they appeared. At length a black spot appears at the top, and spreads till the bubble bursts. 2. If a piece of plain polished glass be placed upon the object-glass of a telescope, and the interval between them be filled up with water, as the glasses are pressed together, the same colours arise at the point of contact, and spread themselves in circular rings round that point in the same order as in the soap-bubble. 3. A convex and concave lens, of nearly the same curvature, being pressed closely together, exhibit rings of colours about the points where they touch. Between the colours there are dark rings, and when the glasses are very much compressed, the central spot is dark. 4. Two pieces of plate-glass wiped clean, and rubbed together, will soon adhere with considerable force, and exhibit various ranges of colours.

From these and other experiments of the like kind, it appears plain, that the colours of bodies depend in some degree upon the thickness and density of the particles that compose them: and if the density or size of the particles in the surface of a body be changed, the colour is likewise changed. Sir Isaac Newton, from a great variety of experiments on light and colours, concluded that every substance in nature, provided it be made sufficiently thin, is transparent : thus gold, when reduced to very thin leaves, transmits a bluish green light. And Mr. Delaval, who, of late years, has done more than almost any other person on the subject, has shewn that colours are exhibited, not by reflected, but by transmitted light. See Memoirs of the Manchester Philosophical Society, Vol. II.

Some portion of light is reflected from every surface of a body, or from every different medium into which it enters. Thus transparent bodies reduced to powder, and water in the shape of froth, appear white, which is no other than a copious reflexion of light from all the surfaces of the minute parts, and from the air interposed between them.

The Rainbow is a meteor in form of a party coloured arch,

or semicircle, exhibited only at the time when it rains, and is always seen in that point of the heavens which is opposite to the sun, and it is occasioned by the refraction and reflexion of his rays in the drops of falling rain. There is likewise, though not always distinctly visible, a secondary, or much fainter rainbow, investing the former at some distance. This beautiful phenomenon has engaged the attention of persons in all ages, but the cause of it could not be ascertained till the discoveries concerning the division of the rays of light into their primary colours, were made.

The doctrine of the different refrangibility of light enables us to give a reason for the size of a bow of each particular colour. All the phenomena of the rainbow are explained in elementary works according to the principles of Sir Isaac Newton. To these works we shall refer, mentioning in this place only the facts upon which the phenomena depend.

1. When the rays of the sun fall upon a drop of rain, aud enter into it, some of them after one reflexion and two refractions, may come to the eye of a spectator, who has his back turned towards the sun, and his face towards the rain. Of the rays of light reflected from a drop of rain coming to the eye, those are called effectual, which can excite a sensation; and they will not be effectual, when they come out of a drop of rain unless they are parallel and contiguous.

When the sun shines upon the drops of rain as they are falling, the rays that come from those drops to the eye of the spectator, after one reflexion and two refractions, produce the primary rainbow, which is never larger than a semicircle.

When the sun shines upon the drops of rain as they are falling, the rays that come from these drops to the eye of a spectator, after two reflexions and two refractions, produce the secondary rainbow. The colours of the secondary rainbow are fainter than those of the primary rainbow, and are ranged in the contrary order. For the primary rainbow is produced by such rays as have been only once reflected, the secondary rainbow is produced by such rays as have been twice reflected.

But at every reflexion some rays pass out of the drop of rain without being reflected, so that the more frequently the rays are reflected, the fewer of them are left to excite a sensation in the eye, therefore the colours of the secondary bow are produced by fewer rays, and consequently will be fainter than the colours of the primary bow.

In the primary bow, reckoning from the outside of it, the colours are ranged in the following order: red, orange, yellow, green, blue, indigo, violet. But in the secondary bow, reckoning from the outside, the colours are reversed, and are ranged: violet, indigo, blue, green, yellow, orange, and red. So that the red, which is the outermost or highest colour in the primary bow, is the innermost or lowest colour in the secondary one.

Of the Eye. Having described the nature of the refraction of rays of light, as performed by different kinds of lenses, we shall shew the effects produced by the several parts of the eye intended for vision.

The eye is of a globular form, and composed of three coats or teguments, one covering the other, and enclosing three different substances called humours. Fig 12 is intended to shew the section of the globe of the eye, the three concentric circles representing the three coats. The external coat, which is represented by the circle ABCDE, is called the sclerotica, of which the front part Cx D is perfectly transparent, and is called the cornea, beyond this towards B and E, it is white, viz. those parts that are called the white of the eye.

The second circle represents the choroides, which encircles the eye entirely, excepting at the small opening usually called the pupil of the eye, that is, the space through which the rays of light pass on to the back of the eye, called the retina. The choroides is distinguished into two parts, of these the larger portion, which is not visible, is peculiarly denominated the choroides; and the front, viz. that part which is blue, or grey, or black, or brown, in different eyes, is called the iris. The iris is composed of a sort of net-work, which contracts or ex

pands according to the force of the light to which the eye is exposed. In a very strong light the iris is enlarged, and the pupil becomes small; in the dusk, or in a darkish room, the iris contracts, and the pupil becomes large, to admit all the light that can be had, the iris being perfectly opake, and admitting no rays through it.

The third coat, represented by the inner circle, is called the retina, so named from its net-work form: it serves to receive the images of objects produced by the refraction of the different humours of the eye. From the hinder part of the eye, but not in the centre, proceeds the optic nerve A, which conveys to the brain the sensation produced upon the retina. Before however we come to speak of the manner of vision, which is dependent on sensation, we shall describe the three humours, as they are called, included within the coats; these are the aqueous, the crystalline and the vitreous humours.

The aqueous is much more of a fluid than the others: it is, in comparison of those, thin and clear like water, and it fills up all the front part of the eye under the cornea; viz. the part distinguished by the letters cx eba: the lens-like part ƒ d is the crystalline humour enveloped by the aqueous humour in front, and by the vitreous humour on the back, which fills up all the space z dfz. Fig. 13 is the representation of the eye as it is seen in the head, in which c A and c B represent the cornea, a c and a b the iris that encircles the pupil r.

Of the way in which Vision is performed. As every point of an object to which the eye is directed, sends out rays in all directions, some rays, from every point on the side next the eye, will fall upon that part of the cornea between a and b; and on passing through the pupil and humours of the eye, they will be converged to as many points on the retina of the eye, and will form on it a distinct picture of the object. This is chiefly done by means of the crystalline humour, which is, and which acts as a completely double convex lens. There is no doubt that objects become visible to the eye, by the sensation which their images thus painted on the retina of the

eye excite, and which is carried by the optic nerve A to the

brain.

Of Spectacles, and their uses. Many persons, it is well known, are defective in the organs of sight, the defect may arise from disease or the malformation of the parts of the eye. Some eyes are too flat, others are too convex. In some, the humours just described lose a part of their transparency, and on that account, a deal of light that enters the eye, is stopped, and lost in the passage, and every object appears dim.

Spectacles are intended to remedy the defects of a short sight, whether it arise from a too great or too small degree of convexity. When the eyes are too flat, convex lenses are made use of; and if the eyes are already too convex, the defect may be remedied by concave glasses.

The property of a convex lens is, as has been shewn, to cause the rays of light to converge sooner than they would without its intervention. In eyes that are flatter than they ought, the rays do not converge when they reach the retina, and on that account objects are seen confused; but by the interposition of a convex lens, they are made to converge at the right place exactly. The property of a concave lens, is to cause the rays to diverge, and it is useful to those persons whose eyes are too round; for to them the rays of light naturally converge to a point before they reach the retina: but as sensation is excited on the retina only, such persons cannot see those objects, the rays from which converge before they reach the retina. They do not however stop, but cross each other, and will excite confused sensations, such as will not convey the idea of distinct vision, because they are not brought to a focus on the retina: a concave lens by causing the rays to diverge, prevents them from converging till they arrive at the retina.

Of Optical Instruments, The microscope is an instrument intended to examine small objects. The human eye cannot distinctly view an object that is nearer to it than about six inches; and since there are very many objects, which at