Page images

it, arising from the very great difference between the magnitude of the undulations of sound and those of light. The undulations of light are incomparably less than the minute aperture, while those of sound are much greater; therefore, when light, diverging from a luminous point, enters the hole, the rays round its edges are oblique, and consequently of different lengths, while those in the centre are direct, and nearly or altogether of the same lengths; so that the small undulations between the centre and the edges are in different phases, that is, in different states of undulation; and therefore the greater number of them interfere, and, by destroying one another, produce darkness all around the edges of the aperture; whereas the central rays, having the same phases, combine and produce a spot of bright light on a wall or screen directly opposite the hole. The waves of air producing sound, on the contrary, being very large compared with the hole, do not sensibly diverge in passing through it, and are therefore all so nearly of the same length, and consequently in the same phase, or state of undulation, that none of them interfere sufficiently to destroy one another; hence all the particles of air in the room are set into a state of vibration, so that the intensity of the sound is very nearly everywhere the same. It is probable, however, that, if the aper

But as

ture were large enough, sound diverging from a point without would scarcely be audible, except immediately opposite the opening. Strong as the preceding cases may be, the following experiment, recently published by Professor Airy, seems to be decisive in favour of the undulatory doctrine. Suppose a plano-convex lens of very great radius to be placed upon a plate of very highly polished metal. When a ray of polarized light falls upon this apparatus at a very great angle of incidence, Newton's rings are seen at the point of contact. the polarizing angle of glass differs from that of metal, when the light falls on the lens at the polarizing angle of glass, the black spot and the system of rings vanish: for although light in abundance continues to be reflected from the surface of the metal, not a ray is reflected from the surface of the glass that is in contact with it, consequently no interference can take place; which proves, beyond a doubt, that Newton's rings result from the interference of the light reflected from the surfaces apparently in contact.

Notwithstanding the successful adaptation of the undulatory system to phenomena, it cannot be denied that an objection still exists in the dispersion of light, unless the explanation given by Professor Airy be deemed sufficient. A sunbeam falling on a prism, instead of being refracted to

a single point, is dispersed, or scattered over a considerable space, so that the rays of the coloured spectrum, whose waves are of different lengths, have different degrees of refrangibility, and consequently move with different velocities, either in the medium which conveys the light from the sun, or in the refracting medium, or in both; whereas it has been shown that rays of all colours move with the same velocity. If, indeed, the velocities of the various rays were different in space, the aberration of the fixed stars, which is inversely as the velocity, would be different for different colours, and every star would appear as a spectrum whose length would be parallel to the direction of the earth's motion, which is not found to agree with observation. Besides, there is no such difference in the velocities of the long and short waves of air in the analogous case of sound, since notes of the lowest and highest pitch are heard in the order in which they are struck. The solution of this anomalous case suggested by Professor Airy from a similar instance in the theory of sound, already mentioned, will be best understood in his own words. "We have every reason," he observes, 'to think that a part of the velocity of sound depends upon the circumstance that the law of elasticity of the air is altered by the instantaneous development of latent heat on compression,

or the contrary effect on expansion. Now, if this heat required time for its development, the quantity of heat developed would depend upon the time during which the particles remained in nearly the same relative state, that is, on the time of vibration. Consequently, the law of elasticity would be different for different times of vibration, or for different lengths of waves; and therefore the velocity of transmission would be different for waves of different lengths. If we suppose some cause which is put in action by the vibration of the particles to affect in a similar manner the elasticity of the medium of light, and if we conceive the degree of development of that cause to depend upon time, we shall have a sufficient explanation of the unequal refrangibility of different coloured rays.' Even should this view be objectionable, instead of being surprised that one dis-crepant case should occur, it is astonishing to find the theory so nearly complete, if it be considered that no subject in the whole course of physicomathematical inquiry is more abstruse than the doctrine of the propagation of motion through elastic media, perpetually requiring the aid of analogy from the unconquerable difficulties of the subject.


It is not by vision alone that a knowledge of the sun's rays is acquired,-touch proves that they have the power of raising the temperature of substances exposed to their action; and experience likewise teaches that remarkable changes are effected by their chemical agency. Sir William Herschel discovered that rays of caloric, which produce the sensation of heat, exist independently of those of light; when he used a prism of flint glass, he found the warm rays most abundant in the dark space a little beyond the red extremity of the solar spectrum, from whence they decrease towards the violet, beyond which they are insensible. It may therefore be concluded that the calorific rays vary in refrangibility, and that those beyond the extreme red are less refrangible than any rays of light. Wollaston, Ritter, and Beckman discovered simultaneously that invisible rays, known only by their chemical action, exist in the dark space beyond the extreme violet, where there is no sensible heat: these are more refrangible than any of the rays of light or heat, and gradually decrease in refrangibility towards the other end of the spectrum, where they cease. Thus the solar spectrum is proved to consist of five superposed spectra, only three of which are

« PreviousContinue »