Part 12 (1/2)

One little reference in the text alone shows that his very nareat work of HELMHOLTZ on physiological optics, HERSCHEL'S labors are not taken account of

It is easy to account for this seeeneration only as an astronomer A study of his ive hih rank indeed, and I trust that the brief suiven 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 hiation of the optical phenos 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 soorous efforts to the study of the phenoht, which are exemplified in the colors of thin and of thick plates The colors of thin plates are ular 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 exaed in concentric circles, and, though others had so produced thes have, ever since the publication of his remarkable work, been known by his naed to suppleht, by supposing that the inconceivably ht are not always equally susceptible of reflection, but that they have periodically recurring ”fits of easy reflection” and of ”easy transh 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 hi clear the true nature of the phenonized 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 filnize 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 s which he was observing This ht as hardly to be detected without the guidance of the definite knowledge acquired in later times, not only vitiated the conclusion froave 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 froenuity in experi as ever He tried the effect of having a polishedthe 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 coht he had found the solution of his probleht which passed through in the red bow Though mistaken, he had presented to the world of science two experiments which have since played very proht, nas 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 rong in his conclusions, and perhaps with less excuse His experienious His philosophizing was distinctly faulty We can see not only that he rong, but exactly where he began to go wrong Yet these papers are full of interest to the physicist, and by no lect into which they have fallen

_Researches on the Dimensions of the Stars_

HERSCHEL exanifying powers, in order to deterood telescope stars present round and pretty uniforular dia, like other objects; but, instead of this, HERSCHEL found that they appeared sly called the disk of light seen in the telescope a spurious disk This singular pheno whether a sht body has an appreciable size, or only ihtness If the first were the case, the apparent size would increase with increased ular dimensions were inappreciable, the apparent size would, on the contrary, di this criterion came in the first years of this century, with the discovery of three s 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 experi 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 na 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 faeneration later

Incidentally the experi 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 hest 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 exaer stars, and found that these stars could be arranged in three general classes or _types_ His results have been verified and extended by other astronoenerally accepted

According to SECCHI, the lucid stars uished 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 soht of a few of the stars of the first lasses of le and to any direction, I had the following analyses:

”The light of _Sirius_ consists of red, orange, yellow, green, blue, purple, and violet _[alpha] Orionis_ contains the sae 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 e, and less yellow in proportion than _Sirius_ _Aldebaran_ contains e and very little yellow _[alpha] Lyrae_ contains reen, blue, and purple”

Here the essential peculiarities of the spectruated by HERSCHEL is pointed out, and if ere to use his observations alone to classify 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 class

HERSCHEL'S immediate object was not classification, and his observations are only recorded in a passing way But the fact reuished 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 shi+ne with differently colored light That of _Arcturus_ and _Aldebaran_, for instance, is as different froht of _Sirius_ and _Capella_ as that of _Mars_ and _Saturn_ is froht of _Venus_ and _Jupiter_”

Of course, no special discovery can be claiood 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 tiht and Heat from the Sun_