Sir William Herschel: His Life and Works

by Edward S. Holden

It is easy to account for this seemingly strange neglect. Herschel is known to this generation only as an astronomer. A study of his memoirs will show that his physical work alone should give him a very high rank indeed, and I trust that the brief summaries, which alone can be given here, will have made this plain.

We may conclude from the time expended, the elaborate nature of the experiments involved, and the character of the papers devoted to their consideration, that the portion of Herschel's researches in physics which interested him to the greatest degree, was the investigation of the optical phenomena known as Newton's rings. In 1792 he obtained the two object-glasses of Huyghens, which were in the possession of the Royal Society, for the purpose of repeating Newton's experiments, and in 1810 he read the last of his three papers on the subject.

Sir Isaac Newton had given some of his most vigorous efforts to the study of the phenomena of interference of light, which are exemplified in the colors of thin and of thick plates. The colors of thin plates are most conveniently studied in the regular form which they present when produced by a thin plate of air, limited on one side by a plane polished surface, and on the other by a spherical surface of long radius, such as the exterior surface of a convex lens, for example. The colors are then arranged in concentric circles, and, though others had so produced them before Newton, these rings have, ever since the publication of his remarkable work, been known by his name.

To explain the phenomena, Newton was obliged to supplement his theory of the corpuscular nature of light, by supposing that the inconceivably minute particles constituting light are not always equally susceptible of reflection, but that they have periodically recurring "fits of easy reflection" and of "easy transmission." This conception, though by no means unphilosophical, seemed to Herschel too artificial and improbable for ready acceptance, and his effort was to supply a more probable explanation.

The developments of optical science have justified Herschel in his objections, but we cannot accord to him must any considerable part in making clear the true nature of the phenomenon. Indeed, it must be recognized that his position was distinctly less advanced than that of Newton. That great philosopher announced the true law governing the relation between the color and the thickness of the film. Herschel did not recognize such a relation. Newton showed exactly how the phenomenon depended upon the obliquity at which it was viewed. Herschel found no place in his theory for this evident variation.

In the series of experiments described in the first paper on this subject, Herschel mistook the locus of a certain set of rings which he was observing. This mistake, though so slight as hardly to be detected without the guidance of the definite knowledge acquired in later times, not only vitiated the conclusion from the experiments, but gave an erroneous direction to the whole investigation. To him these experiments proved that Newton's conception of a periodic phenomenon was untenable. Thus cut loose from all hypothesis, his fertility in ideas and ingenuity in experimentation are as striking as ever. He tried the effect of having a polished metal as one of the surfaces limiting the thin plate of air. Observing the so-called "blue bow" of Newton at the limit of total reflection in a prism, he was led to the discovery of its complement, the "red bow" by refraction. Here he thought he had found the solution of his problem, and attributed the rings to the reflection of the light which passed through in the red bow. Though mistaken, he had presented to the world of science two experiments which have since played very prominent parts in the undulatory theory of light, namely, the rings formed upon polished metal, and the bands produced by a thin plate near the critical angle.

As in his later researches upon the nature of radiant heat, he was wrong in his conclusions, and perhaps with less excuse. His experiments were skilfully devised and most ingenious. His philosophizing was distinctly faulty. We can see not only that he was wrong, but exactly where he began to go wrong. Yet these papers are full of interest to the physicist, and by no means deserve the neglect into which they have fallen.

Researches on the Dimensions of the Stars.

Herschel examined a number of bright stars, using extremely high magnifying powers, in order to determine whether the stars have sensible dimensions. In a good telescope stars present round and pretty uniformly illuminated disks. If these disks really represent the angular diameter of the stars, they should admit of magnifying, like other objects; but, instead of this, Herschel found that they appeared smaller as the telescopic power was increased. He accordingly called the disk of light seen in the telescope a spurious disk. This singular phenomenon gave its discoverer a ready criterion for determining whether a small bright body has an appreciable size, or only impresses the sense of sight by virtue of its intrinsic brightness. If the first were the case, the apparent size would increase with increased magnifying power, while, if the angular dimensions were inappreciable, the apparent size would, on the contrary, diminish with additional magnifying. An occasion for using this criterion came in the first years of this century, with the discovery of three small planets having orbits lying between those of Mars and Jupiter. Herschel gave the name Asteroids to these bodies. As the appropriateness of this term had been violently assailed, the discovery of Juno, in 1804, the third one of the group, led to a careful experimental study of the defining power of the telescope used, and of the laws governing the phenomena of spurious disks.

With a telescope of about nine inches in aperture, Herschel found that if Juno subtended an angle greater than a quarter of a second of arc, a certain indication of the fact would have shown itself in the course of the experiments. This conclusion was a justification of the name Asteroid, since the appearance of the new planet was strictly stellar. On other grounds, a better name might have been selected.