Sir William Herschel: His Life and Works Part 12

One little reference in the text alone shows that his very name was not unknown. Even in the great work of HELMHOLTZ on physiological optics, HERSCHEL'S labors are not taken account of.

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-gla.s.ses 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 const.i.tuting 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 pa.s.sed 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 a.s.sailed, 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.

In the paper giving the results of the experiments, the phenomena of the spurious disks are very completely described; but they did not attract the attention which they deserved, and they only became an object of especial interest to students of physics when they were again studied by the famous German optician FRAUNHOFER, a generation later.

Incidentally the experiments are of interest, as yielding us a measure of the excellence of HERSCHEL'S telescopes, and a measure which is quite independent of the keenness of his vision. From them we may be sure that the efficiency of the nine-inch mirror used was not sensibly less than that of the highest theoretically attainable excellence. In this connection, too, we may refer to the _Philosophical Transactions_ for 1790, pp. 468 and 475, where HERSCHEL gives observations of both _Enceladus_ and _Mimas_ seen in contact with the ball of _Saturn_.

I have never seen so good definition, telescopic and atmospheric, as he must have had on these occasions.

_Researches on the Spectra of the Fixed Stars._

The spectroscope was applied by SECCHI to the study of the spectra of the fixed stars visible to the naked eye in the years 1863 to 1866.

He examined the nature of the spectrum of each of the larger stars, and found that these stars could be arranged in three general cla.s.ses or _types_. His results have been verified and extended by other astronomers, and his cla.s.sification has been generally accepted.

According to SECCHI, the lucid stars may be separated into three groups, distinguished by marked differences in their spectra. SECCHI'S Type I.

contains stars whose spectra are like those of _Sirius_, _Procyon_, and _[alpha] Lyrae_; his Type II. stars like _Arcturus_ and _Aldebaran_; his Type III. stars like _[alpha] Orionis_.

HERSCHEL also made some trials in this direction. In the _Philosophical Transactions_ for 1814 (p. 264), he says:

"By some experiments on the light of a few of the stars of the first magnitude, made in 1798, by a prism applied to the eye-gla.s.ses of my reflectors, adjustable to any angle and to any direction, I had the following a.n.a.lyses:

"The light of _Sirius_ consists of red, orange, yellow, green, blue, purple, and violet. _[alpha] Orionis_ contains the same colors, but the red is more intense, and the orange and yellow are less copious in proportion than they are in _Sirius_. _Procyon_ contains all the colors, but proportionately more blue and purple than _Sirius_. _Arcturus_ contains more red and orange, and less yellow in proportion than _Sirius_. _Aldebaran_ contains much orange and very little yellow. _[alpha] Lyrae_ contains much yellow, green, blue, and purple."

Here the essential peculiarities of the spectrum of each of the stars investigated by HERSCHEL is pointed out, and if we were to use his observations alone to cla.s.sify these stars into types, we should put _Sirius_ and _Procyon_ into one type of stars which have "all the colors" in their spectra; _Arcturus_ and _Aldebaran_ would represent another group of stars, with a deficiency of yellow and an excess of orange and red in the spectrum; and _[alpha] Orionis_ would stand as a type of those stars with an excess of red and a deficiency of orange.

_[alpha] Lyrae_ would represent a sub-group of the first cla.s.s.

HERSCHEL'S immediate object was not cla.s.sification, and his observations are only recorded in a pa.s.sing way. But the fact remains that he clearly distinguished the essential differences of the spectra of these stars, and that he made these observations in support of his statement that the fixed stars, "like the planets, also shine with differently colored light. That of _Arcturus_ and _Aldebaran_, for instance, is as different from the light of _Sirius_ and _Capella_ as that of _Mars_ and _Saturn_ is from the light of _Venus_ and _Jupiter_."

Of course, no special discovery can be claimed for him on these few instances. We can see, however, a good example of the manner in which he examined a subject from every side, and used the most remote evidence exactly in its proper place and time.

_Researches on the Variable Emission of Light and Heat from the Sun._

It is certainly a remarkable fact that HERSCHEL was the first observer to recognize the real importance of the aperture or diameter of a telescope. Before his time it was generally a.s.sumed that this element only conditioned the amount of light transmitted to the eye, or, in other words, merely determined the brightness of the image. Hence the conclusion that if an object is sufficiently bright, the telescope may be made as small as desired without loss of power. Thus, in observing the sun, astronomers before HERSCHEL had been accustomed to reduce the aperture of their telescopes, in order to moderate the heat and light transmitted. SCHEINER, it is true, nearly two centuries before the time we are considering, had invented a method for observing the sun without danger, still employing the full aperture. This was by projecting the image of the sun on a white screen beyond the eye-piece, the telescope being slightly lengthened. For special purposes this ingenious method has even been found useful in modern times, though for sharpness of definition it bears much the same relation to the more usual manner of observing, that a photographic picture does to direct vision.

Although HERSCHEL saw the advantages of using the whole aperture of a telescope in such observations, the practical difficulties in the way were very great. We have noted his attempts to find screens which would effectively cut off a large portion of the heat and light without impairing vision, and have considered, somewhat in detail, the remarkable discoveries in radiant heat to which these attempts led him.

His efforts were not unsuccessful. A green gla.s.s smoked, and a gla.s.s cell containing a solution of black writing ink in water--were found to work admirably.

Thus provided with more powerful instrumental means than had ever been applied to the purpose, HERSCHEL turned his attention to the sun. In a very short time he exhausted nearly all there was to be discovered, so that since the publication of his last paper on this subject, in 1801, until the present time, there has been but a single telescopic phenomenon, connected with the physical appearance of the sun, which was unknown to HERSCHEL. That phenomenon is the frequent occurrence of a darker central shade or kernel in large spots, discovered by DAWES about 1858.

HERSCHEL, though observing a hundred and ninety years after the earliest discovery of sun spots, seems to have been the first to suspect their periodic character. To establish this as a fact, and to measure the period, was left for our own times and for the indefatigable observer SCHWABE. The probable importance of such a period in its relation to terrestrial meteorology was not only clearly pointed out by HERSCHEL, but he even attempted to demonstrate, from such data as were obtainable, the character of this influence.

Perhaps no one thing which this great philosopher has done better exhibits the catholic character of his mind than this research. When the possible connection of solar and terrestrial phenomena occurred to him as a question to be tested, there were no available meteorological records, and he could find but four or five short series of observations, widely separated in time. To an ordinary thinker the task would have seemed hopeless until more data had been collected. But HERSCHEL'S fertile mind, though it could not recall lost opportunities for solar observations, did find a subst.i.tute for meteorological records in the statistics of the prices of grain during the various epochs.

It is clear that the price of wheat must have depended upon the supply, and the supply, in turn, largely upon the character of the season.

The method, as ingenious as it is, failed in HERSCHEL'S hands on account of the paucity of solar statistics; but it has since proved of value, and has taken its place as a recognized method of research.

_Researches on Nebulae and Cl.u.s.ters._

When HERSCHEL first began to observe the nebulae in 1774, there were very few of these objects known. The nebulae of _Orion_ and _Andromeda_ had been known in Europe only a little over a hundred years.

In 1784 MESSIER published a list of sixty-eight such objects which he had found in his searches for comets, and twenty-eight nebulae had been found by LACAILLE in his observations at the Cape of Good Hope. In the mere discovery of these objects HERSCHEL quickly surpa.s.sed all others.

In 1786 he published a catalogue of one thousand new nebulae, in 1789 a catalogue of a second thousand, and in 1802 one of five hundred. In all he discovered and described two thousand five hundred and eight new nebulae and cl.u.s.ters. This branch of astronomy may almost be said to be proper to the HERSCHELS, father and son. Sir JOHN HERSCHEL re-observed all his father's nebulae in the northern hemisphere, and added many new ones, and in his astronomical expedition to the Cape of Good Hope he recorded almost an equal number in the southern sky.

Of the six thousand two hundred nebulae now known the HERSCHELS discovered at least eight-tenths. The mere discovery of twenty-five hundred nebulae would have been a brilliant addition to our knowledge of celestial statistics.

HERSCHEL did more than merely point out the existence and position of these new bodies. Each observation was accompanied by a careful and minute description of the object viewed, and with sketches and diagrams which gave the position of the small stars in it and near it.[36]

As the nebulae and cl.u.s.ters were discovered they were placed in cla.s.ses, each cla.s.s covering those nebulae which resembled each other in their general features. Even at the telescope HERSCHEL'S object was not discovery merely, but to know the inner const.i.tution of the heavens.

His cla.s.ses were arranged with this end, and they are to-day adopted.

They were:

CLa.s.s I. "Bright nebulae (288 in all).

II. "Faint nebulae (909 in all).

III. "Very faint nebulae (984 in all).

IV. "Planetary nebulae, stars with burs, with milky chevelure, with short rays, remarkable shapes, etc. (79 in all).

V. "Very large nebulae (52 in all).

VI. "Very compressed and rich cl.u.s.ters of stars (42 in all).

VII. "Pretty much compressed cl.u.s.ters (67 in all).

VIII. "Coa.r.s.ely scattered cl.u.s.ters of stars" (88 in all).

The lists of these cla.s.ses were the storehouses of rich material from which HERSCHEL drew the examples by which his later opinions on the physical conditions of nebulous matter were enforced.

As the nebulae were discovered and cla.s.sified they were placed upon a star-map in their proper positions (1786), and, as the discoveries went on, the real laws of the distribution of the nebulae and of the cl.u.s.ters over the surface of the sky showed themselves more and more plainly. It was by this means that HERSCHEL was led to the announcement of the law that the s.p.a.ces richest in nebulae are distant from the Milky Way, etc.

By no other means could he have detected this, and I believe this to have been the first example of the use of the graphical method, now become common in treating large ma.s.ses of statistics.