Page images
PDF
EPUB

power of the prism, is evident from this experiment. If the rays of the sun fall upon a prism, and be thereby refracted into their different colours, and the rays of any particular colour, as red or blue, be separately transmitted through a second prism, they are no farther separable. They are indeed refrangible, and may be refracted in a different direction; but the colour is never changed. If the colour depended upon any operation of the prism, and were not inherent in the rays of light, the second prism would separate the rays as well as the first. This, however, never happens: the red rays continue red, and the blue rays, blue, after transmission through the second prism. But we shall say more on this subject, when we come to consider the colours of natural bodies.

If the solar rays be equally refracted by two prisms of different kinds of glass, the dispersion of the rays of any colour, effected by the one may be much greater than that by the other, and the coloured spectrum or image of the sun formed by the one will be much longer than that formed by the other. It is found, that in equal angles of mean refractions, a prism of white flint glass, in the composition of which there is a considerable quantity of lead, will disperse the component colours of light, much more than a prism which abounds with alkaline salts.

REFLEXION OF LIGHT.

WHEN the rays of light fall upon any body, instead of being transmitted through its pores, they are copiously reflected back; not by impinging on the solid parts of the body and rebounding again like an elastic ball, but by some power that is equally diffused over the surface of the body, and very probably of a re

pulsive kind; as we know, that where the attraction of cohesion ends a power of repulsion begins. And many of them are reflected from the second surface of the glass, after having entered its There pores. are many reasons, which induce us to believe, that light is reflected in the manner above mentioned. If it be incident upon glass in an obliquity of 41° or 42o, when air is in contact with the second surface, it is wholly reflected. But in this degree of obliquity, if water be in contact with the second surface of the glass, the rays will be transmitted through it, until the obliquity be increased to 45° or 46°. Now it cannot be supposed, that in one degree of obliquity, the rays of light should meet with nothing but pores to transmit them, and in another degree, with nothing but solid particles to reflect them: nor that they should meet with more particles in air than in water to reflect them. We find also that the reflexion from the first surface is less copious than that from the second; and in general, the rarer that any transparent medium is, that is in contact with the second surface of glass, the more copious is the reflexion. If glass, water, air, and a vacuum be successively applied to the second surface of glass, the rays of light will be more copiously reflected in each succeeding experiment than in the former. Now it is absurd to suppose that light should meet with more solid particles to reflect it, the rarer the medium is, which causes the reflection; and that it should meet with more solid particles in a vacuum, where there are none at all to reflect it, than in glass or water: and yet the reflexion from it is most copious: and from air more copious than from water, and from water, than from glass.

Besides, Sir Isaac Newton found that the rays of light were as differently reflexible as they were refrangible; and that those that were most refrangible, were also most reflexible. Hence it is, that the atmosphere appears to be blue. When all the rays of other colours are transmitted through the atmosphere near to the top of it, where it is exceedingly rare, the blue rays alone, being most easily reflected, are brought to the eye. Blue rays will be reflected when others are transmitted.

If the object-glass of a refracting telescope, of considerable focal length, were laid upon a plane glass, the light falling upon the thin plate of air between the glasses will be alternately transmitted and reflected, at different distances from the center, or point of contact. In the center, where the lens and glass are in contact, the light will be wholly transmitted, and thereby cause the central spot to appear black. At a small distance from thence the light will be reflected in various coloured rings, and so on to a considerable distance from the center. Should these distances be taken in arithmetical progression, as 0, 1, 2, 3, 4, 5, &c. then at the distances 0, 2, 4, 6, 8, &c. the light will be transmitted, and at the distances 1, 3, 5, 7, 9, &c. it will be reflected. The same appearances are observed in bubbles of water, rendered tenacious by a little soap. But in these, the concentric rings of coloured light reflected, continually expand, as the bubble grows thinner by the continual subsiding of the water, until the bubble breaks.

These phenomena prove, not only that some rays may be transmitted while others are reflected; but also, that the reflexion depends upon the thickness of the bubble, and of the plate of air between the glasses.

But that which proves, beyond all contradiction, that the rays are differently reflexible, according to their refrangibility, is the following experiment.

After the rays of the sun are separated by a prism, let them be received by another, which, by its revolution on its axis, will begin to reflect the violet rays, while it transmits the rays of other colours, and reflects the red rays last of all.

This may be adequately accounted for, upon the supposition, that the violet rays are of the least magnitude, and therefore, coming with the least momentum, are most easily reflected by the repulsive force of the medium on which they are incident.

But that the transmission of rays of one colour, at the same place, where others of a different colour were reflected, should depend upon the different thickness of the medium through which they pass, or of that which reflects them, is not so easily accounted for. Sir Isaac Newton, to whom we are indebted for all that we know of light, supposes that we have sufficient foundation from this phenomenon to conclude, that the rays of light, in their passage from the sun, are, at certain intervals, disposed to be alternately transmitted and reflected. After all the experiments which he made upon light, that prince of philosophers confessed, that he was not able to account for the mode of operation which nature uses in producing these phenomena. Yet, even the hypotheses of so great a genius are not to be lightly rejected. His solution of this appearance is consonant to that simplicity and regularity, which we discover in all the works of nature. His opinion was, that if the rays of light were incident on any medium, in a fit of easy transmission, they were accordingly transmitted; but if in a fit of easy reflexion, they were reflected.

1

Every ray of light, in its passage through any refracting surface, is put into a certain transient state, which at certain intervals disposes it to be easily transmitted or reflected. These returns of disposition to be easily transmitted or reflected, he called fits of easy transmission and reflection; and the space it passes over between these returns he denominated the intervals of the fits. These intervals he also found to be different in the different colours, as he observed the same ray to be in a fit of easy transmission at both surfaces of the lens, when incident upon it at a convenient thickness; whereas, rays of another colour were not transmitted, unless they were incident on the glass, where it was of a different thickness. He supposed that these dispositions may be communicated to the rays, by some elastic medium, through which they pass, the motion of whose particles may be quicker than that of the rays of light. So that when a ray of light is in that part of the vibration of this elastic medium, which conspires with its own motion, it may be transmitted through any body on which it should then happen to be incident; but that it would be reflected, if it should then be in that part of the vibration of this medium, which is contrary to its own motion.

COLOURS OF NATURAL BODIES.

As the rays of light, falling on the thin plate of air under the object-glass of a telescope, are variously disposed to be transmitted or reflected, according to the different thicknesses of the glass; so when they fall on the surfaces of natural bodies they will be variously reflected or transmitted, by the pores filled with air of various diameters, and according to the different texture of the bodies, or the size of the particles of light.

[graphic]
« PreviousContinue »