delicate balance-beam nicely suspended, by which means, a slight motion being given to the beam, it was inferred that the light thus collected, had a sensible momentum. From the weight of the beam, and from the motion which was communicated to it by the impulse of light, also from the well known velocity of light, it was calculated, that the matter contained in the light which was thrown upon the end of the above-mentioned beam, during one second of time, and which was collected from a reflecting surface, of about 4 square feet, amounted to no more than one 1200 millionth part of a grain. But an uninterrupted sensation of vision, may be produced by a discontinuous series of impressions of the luminous particles; as a light rapidly moved across a hole in a dark screen, makes the hole appear constantly luminous. Therefore it was further computed, that a particle of light projected every tenth of a second from each point of a luminous surface, was sufficient for uninterrupted vision. But in one tenth of a second, light moves through twenty thousand miles; hence this may be the interval between the successive links in the luminous ray, permitting other particles to cross without chance of mutual obstruction. Another step led the same theorists to measure the waste of luminous matter, which the sun would experience in a day, a quantity which they found to amount to 2 grains; constituting only 670 pounds, avoirdupois, in 6000 years.

It is difficult to say, whether the above experiments or deductions are most absurd. A delicate balance, that for example made by Ramsden, for the

EXPERIMENTS ON THE RADIATION OF LIGHT. 19

Royal Society, is so easily disturbed by heat, that the one arm will elongate and cause its scale to descend, when it receives the radiation of heat from the body of an experimenter, more directly than the other arm. Hence, by his moving a little to the right or to the left, in front of the beam, its truth will be impaired. Therefore, the heating influence of the sunbeam, on the air and the balance, in the experiments recorded in Priestley's History of Light, Period vi. Section 1. Chap. iii. was the proper and adequate cause of the elongation of the beam, and of the apparent weight of the luminous particles. In fact, of two closely corked globes of water accurately poised at the same temperature, in a delicate balance, if one be heated and replaced in the scale, it will preponderate by the elongation of its end of the beam. Neither these experiments, nor any other that I have seen described, justify us in concluding light to be an emission of material particles, however small. On the contrary, Dr. Thomas Young, Secretary of the Board of Longitude, has shown, by new experiments and researches, no less ingenious than precise, that light does not consist in the emanation of particles, but in the undulatory movements of a luminiferous ether, which pervades the Universe, rare and elastic in a high degree. He further shows, that this theory of the nature of light, long ago embraced by Huyghens, was at one time contemplated by Newton, with no unfavourable eye. “Were I," says Newton, "to assume an hypothesis, it should be this, if propounded more generally, so as not to determine what light is, further than that it is some

thing or other capable of exciting vibrations in the ether; for thus it will become so general, and comprehensive of other hypotheses, as to leave little room for new ones to be invented. (Birch. iii. 249, Dec. 1675.)

When two undulations, from different origins, coincide either perfectly or very nearly in direction, their joint effect is a combination of the motions belonging to each. When they differ in frequency, they produce different colours, as the different frequency of aerial undulations, produces different musical tones. When two series of equal frequency coincide exactly in point of time, it is obvious that the united velocity of the particular motions must be greatest; and also, that it must be smallest, and if the undulations be of equal strength, totally destroyed, when the time of the greatest or forward motion, belonging to one undulation, coincides with that of the greatest retrograde motion of the other. In intermediate states, the joint undulation will be of intermediate strength. It is well known, that a similar cause produces, in sound, that effect which is called a beat in music; two series of acrial undulations of nearly equal magnitude, alternately cooperating, and destroying each other, according as they coincide more or less perfectly in the times of performing their respective motions.*

In a paper read before the Royal Society, July 1, 1802, Dr. Young laid down the following optical law; "that wherever two portions of the same light

• Dr. Young, Phil. Trans. Nov. 12, 1801.

LIGHT INDEPENDENT OF THE SUN.

21

arrive at the eye by different routes, either exactly or very nearly in the same direction, the light becomes most intense, when the difference of the routes is any multiple of a certain length, and least intense in the intermediate state of the interfering portions and this length is different for light of dif ferent colours." This important law, the basis of a new and admirable theory of light, has been since fully adopted by M. Fresnel, and M. Arago, who have enforced and illustrated it by many fine researches. It demonstrates incontestibly the separate existence of a luminiferous ether, which may be made to undulate, not only by the sun and other permanent foci of vibration; but by a vast number of other causes, such as the friction or gentle heating of many mineral solids, and by several chemical actions, independent of combustion. Hence we see that this ether, being indispensable to the operation of every luciferous impulse, being in fact the substratum or subject matter of light, as air is of sound in general, must necessarily have had a precedent and independent existence, as Moses has declared in his narrative of creation. That luciferous impressions may be produced without any intercourse with the sun, is established by many facts. The phosphorescence of minerals buried since the origin of things in the bowels of the earth; the electric light caused by friction, metallic contact, or the volition of the electric eel; the luminousness of many insects, worms, and marine mollusca in the living state, and of the fibres of animals and vegetables, after death; the lucid points of the moon's disc, where the sunbeams

do not fall, all attest that light may exist without solar excitation. The luciferous action of dead fish may be transferred to water, may be brightened by adding a certain quantity of saline matter to the solution, darkened by a greater quantity, relumed by moderate dilution, and finally destroyed by heat or excess of water. It is very curious to see the luminous water, gradually becoming dim with the addition of salt, till the light finally disappears; and instantly bursting forth again from absolute darkness, by a certain dilution with water. Thus we may fill a wine glass with light. In all these cases, the luminous phenomena result from the vibratory impression communicated with greater or less force to the ethereous medium. These vibrations may be multiplied or diminished, by various causes, apparently very slight, and very enigmatical. In our further inquiries into the nature of light, we shall find that, if the length of a luminous undulation be less than one sixty thousandth of an inch, it becomes incapable of exciting the sensibility of our visual organs; but chemical experiments prove, that much shorter and more rapid undulations of the luminiferous ether exist beyond the luminous verge of the spectrum, which are still essentially light, though imperceptible to our visual orb. How unphilosophical therefore to infer, the absolute want or non-existence of light, whenever our purblind optics cannot discern it? And since we know that the luciferous ether be thrown into visible luminous undulation, without the sun or stars, and into invisible luminous undulation by the sun and stars, what reason have we to

may